enclosed in a canal by a strong band, the anterior annular ligament, and their surfaces are invested by a synovial membrane, which facilitates their movements to and fro beneath that ligament; as they pass downwards in front of the fingers they are enclosed in a strong fibrous sheath lined by a synovial membrane, and the tendon of the superficial flexor is pierced by the deep flexor, so that the latter may reach the third phalanx into which it is inserted. Four rounded muscles, the lumbricales, arise in the palm from the deep flexor tendons, turn round the radial borders of the first phalanges, and are inserted one into the extensor tendon on the dorsum of each finger; these muscles bend the first phalanges on the metacarpal bones, but from their insertion into the extensor tendons they also extend the second and third phalanges on the first; as they are much used in playing stringed instruments, they have been called "fiddlers' muscles." The fingers are extended or straightened by muscles inserted into the back of the second and third phalanges; the extensor muscles descend from the back of the fore-arm,-one, the common extensor, subdivides into four tendons, one for each finger, but in addition the index and little have each a separate extensor muscle, the tendon of which joins that of the common extensor. The index finger possesses more independent movement than the other digits-hence its more frequent use as a "pointer;" the extensor tendons of the little and ring fingers are usually united together, so that these digits are associated in their movements. Abduction and adduction of the fingers are caused by seven small muscles situated in the intervals between the metacarpal bones,-hence called interossei; four of these lie on the back of the hand, three on its palmar surface; they are inserted into the sides of the first phalanges, and either pull the fingers away from a line drawn through the middle finger or approximate them to that line. Too great abduction is checked by the transverse metacarpal ligament. The human hand is a perfect instrument of prehension; not only can the individual fingers be bent into hooks, but the thumb can be thrown across the front of the palm, so that it can be opposed to the several fingers, and objects can therefore be grasped between it and them; but further, this power of opposing the thumb permits objects to be held in the palm of the hand, which may be hollowed into a cup or made to grasp a sphere. The movements of the joints are indicated on the surface of the palm by tegumentary folds,-an oblique fold for the thumb, and two oblique folds for the metacarpo-phalangeal joints of the fingers; the joints of the second and third phalanges are also marked on the surface by folds.

JOINTS AND MUSCLES OF THE LOWER LIMB. The innominate bones are connected to the spinal column by the sacro-iliac joints and the sacro-sciatic ligaments. The Sacro-iliac Joint is between the side of the sacrum and the internal surface of the ilium, the articular surfaces of which bones are covered by cartilage, and connected together by short, strong ligaments. The sacro-sciatic ligaments stretch from the side of the sacrum and coccyx to the spine and tuberosity of the ischium. The two innominate bones are also connected together at the pubic symphysis, which is an amphiarthrodial joint. The sacroiliac joints and pubic symphysis permit only slight movement; that at the former is around an imaginary axis, drawn transversely through the second sacral vertebra, which allows the base of the sacrum to be thrown forward and its apex backward in the stooping position of the body; but too great movement backward of the apex is checked by the sacro-sciatic ligaments. As the weight of the trunk, or of what may be carried in the arms or on the back, is transmitted through the haunch-bones to the lower limbs,

the sacro-iliac ligaments require to be of great strength, because the sacrum, and with it the entire trunk, are suspended by them on the two innominate bones.

The Hip Joint is a ball-and-socket joint; the ball is the Hip joint. head of the femur, and the socket the cup-shaped acetabulum in the haunch bone, the depth of the cup being in creased by a ligament which is attached around the brim. A large capsular ligament, which is especially strong in front, encloses the articular surfaces. The ligament is lined by a synovial membrane, which also invests the neck of the thigh bone. Within the joint is the round or suspensory ligament attached to the head of the thigh bone and to the sides of the depression at the bottom of the acetabulum. Whilst the hip joint possesses considerable mobility, it has much more stability than the shoulder, owing to the acetabulum being deeper than the glenoid fossa, and the greater strength and tension of the fibres of its capsular ligament. The muscles which move the thigh at the hip joint are situated either behind the joint, where they form the fleshy mass of the buttock, or at the front and the inner side of the thigh. They are inserted either into the femur or fascia lata, and the great and small trochanters serve as their principal surfaces of attachment. The thigh can be bent on the abdomen by the action of the psoas, iliacus, and pectineus, which lie in front of the joint; it can be extended or drawn into line with the trunk by the gluteus-maximus and medius; it can be abducted or drawn away from the opposite thigh by the gluteus maximus, medius, and minimus, which muscles are of large size, and form the fleshy mass of the buttocks. It can be adducted or drawn to touch its fellow, or, if slightly bent, drawn in front of its fellow, by the adductor longus, brevis, and magnus, which muscles are inserted into the linea aspera, and form the fleshy mass on the inner side of the thigh; and by the pectineus and quadratus femoris. It can be rotated outwards by the obturator and gemelli muscles, the gluteus maximus, pyriformis, and quadratus femoris; and rotated inwards by the gluteus medius, minimus, and tensor fasciæ femoris. In standing erect the hip joints are fully extended, and the mechanical arrangements in and around these articulations are such as to enable them to be retained in the extended position with but a small expenditure of muscular power. As the weight of the body in the erect attitude falls behind the joints, the strong anterior fibres of their capsular ligaments are made tense, and the extended position of the joints is preserved. So long as the centre of gravity falls within the basis of support of the body, i.e., the space between the two feet when standing on both legs, the body will not fall. If the body is made to lean forward, then the capsular ligament is no longer tense, and the gluteal muscles are put in action to re-extend the trunk on the thigh, and prevent it from falling forward; if the body is made to lean to one side or the other, the round ligament is made tense, or the strong ilio-tibial band of the fascia lata of the thigh, which stretches from the ilium to the tibia, is put on the stretch, and falling sideways is prevented. When, in standing erect either on one or both feet, the balance of the body is disturbed, then various muscles both of the trunk and lower limb are brought into action to assist in preserving the erect position. In the erect position the weight of the trunk is transmitted through the acetabula to the heads of the thigh-bones, but the position and connections of the round ligament enable it to suspend that portion of the trunk the weight of which is thrown upon it, and to distribute the weight over the head of the femur.

The Knee is the largest and most complicated joint in Knee. the body. It consists of the femur, tibia, and patella. The patella moves up and down the trochlear surface of

Joints of

foot.

the femur, whilst the condyles of the femur roll upon the semilunar cartilages and articular surfaces of the tibia. A powerful investing ligament encloses the articular surfaces. This ligament is subdivided into bands, one on each side of the joint-the internal and external lateral ligaments-a posterior and an anterior. The anterior extends from the patella to the anterior tubercle of the tibia, and serves both as a ligament and as the tendon of insertion of the extensor muscles of the leg. Within the investing ligament two interarticular or crucial ligaments pass from the inter-condyloid fossa to the upper surface of the tibia; and interposed between the tibia and femoral condyles are two menisci, which from their shape are called the semilunar cartilages. The synovial membrane not only lines the investing ligaments, but covers the front of the femur for some distance above the trochlea, and forms folds or pads within the joint itself, which in certain movements are interposed between the articular surfaces of the bones. The movements at this joint are those of flexion and extension. The flexors are the three great muscles on the back of the thigh, called the ham-strings; they all arise from the ischial tuberosity, and are inserted-the biceps into the head of the fibula, the semi-tendinosus and semi-membranosus into the upper end of the tibia. The extensors form the fleshy mass on the front and outer side of the thigh; one muscle, the rectus, arises from the ilium-the others, the vasti, from the shaft of the femur; and they are all inserted by a powerful tendon into the patella, and through the anterior ligament of the knee into the tibia. The patella is indeed a sesamoid bone, developed in the tendon of these muscles (Fig. 18). The knee can be bent so that the calf can touch the back of the thigh, and in this position the patella is drawn down in front of the joint, as in kneeling. The articular surface of the patella is divided into seven areas or facets, and in passing from the bent to the extended position of the joint, these facets come successively into contact with the articular surface of the femur, until, when the leg is fully extended on the thigh, the whole of the patella is raised above the femoral trochlea, except the lowest pair of narrow facets. It is in order to provide a smooth surface for the patella in this position that the synovial membrane of the joint covers the front of the lower end of the femur. At the commencement of flexion a slight rotation inwards of the leg and foot takes place through the action of the sartorius, gracilis, and semi-tendinosus, which are inserted close together into the tibia; whilst the extensor muscles cause, at the completion of extension, a slight rotation outwards of the leg and foot. The movements of flexion and extension are not simply in the antero-posterior plane, but along oblique paths which are determined by the screwed configuration of the femoral condyles. In complete extension of the leg the joint is "screwed home;" and as this position is necessary for the preservation of the erect attitude, the lateral, the posterior, and the anterior crucial ligaments are then all tense, to prevent displacement of the bones. The muscles which rotate the leg and foot inwards initiate the act of flexion by unlocking the joint.

The Tibio-fibular Joints are found between the upper and lower ends of the bones, and in addition a strong interosseous membrane fills up the interval between their shafts. The movement between the two bones is almost inappreciable.

The Ankle Joint is formed by the convex upper and the ankle and lateral surfaces of the astragalus fitting into the concavity forried by the lower end of the tibia and the two malleoli. An investing ligament, lined by synovial membrane, encloses the joint; the lateral portions of this ligament form distinct bands, and are much stronger than the anterior and posterior fibres. A diarthrodial joint also

exists between the astragalus and os calcis, between which bones a powerful interosseous ligament passes. Between the astragalus and scaphoid, and the os calcis and cuboid, important diarthrodial joints are found, which are enclosed by ligamentous bands. The remaining tarsal bones are connected together usually by dorsal, plantar, and interosseous ligaments, and a similar mode of union is found between the distal row of tarsal bones and the metatarsals, except between the great toe and ento-cuneiform, where there is no interosseous ligament. The four outer metatarsals are also connected at their proximal ends by distal, plantar, and interosseous ligaments; and further, a transverse metatarsal ligament passes between the distal ends of all the metatarsal bones. The metatarsal bones articulate with the phalanges, and the phalanges with each other, in a similar manner to that described in the corresponding bones of the hand.

At the ankle joint movements of flexion and extension take place. The dorsum of the foot is bent towards the front of the leg by the direct action of the muscles on the front of the leg, more especially the tibialis anticus, inserted into the ento-cuneiform and metatarsal of great toe, and the peroneus tertius, inserted into the metatarsal of little toe; the opposite movement, the so-called extension of the foot, is due to the action of the gastrocnemius and soleus, the great muscles of the calf of the leg, which are inserted by the Tendo Achillis into the posterior prominence of the os calcis or heel. This movement is made at every step in walking or running, and the great size of the calf-muscles is in relation to their use in the act of progression. foot cannot, however, be drawn so far back as to be brought into direct line with the leg. In standing erect the foot is at right angles to the axis of the leg, the astragalus is locked in between the two malleoli, and the fibres of the lateral ligaments are tense, so as to check movement forwards or backwards, and prevent displacement.

The

Between the several bones of the tarsus a certain amount of gliding is permitted, more especially between the os calcis and cuboid and the astragalus and scaphoid, so that it is possible to invert or evert the foot, i.e., to raise its inner or outer borders from the ground. The inversion is performed by the tibialis anticus and by the tibialis posticus, which latter is inserted into the scaphoid bone; the eversion by the peroneus longus and brevis muscles, situated on the outer side of the leg, the tendons of which pass behind the outer malleolus, the brevis to be inserted into the metatarsal bone of the little toe, the longus into the plantar surface of the metatarsal bone of the great toe. The individual toes are bent on the sole by the action of the flexor muscles inserted into the plantar surface of the phalanges, and they are straightened by the extensor muscles inserted into their dorsal surfaces; the toes also can be drawn asunder or abducted, and drawn together or adducted, chiefly by the action of the interossei muscles. The hallux or great toe is the most important digit; a line prolonged backwards through it to the heel forms the proper axis of the foot, and the sole chiefly rests upon the pads of integument situated beneath its metatarso-phalangeal joint and the heel. The hallux is much more restricted in its movements than the thumb: the configuration of its tarso-metatarsal joint and the attachment of the transverse metatarsal ligament prevent the great toe from being thrown across the surface of the sole as the thumb is thrown across the palm in the movement of opposition; an object can, however, be grasped between the hallux and second toe by the action of its adductor muscles, and persons can be trained to write with a pen or pencil held in this position.

The act of walking consists in the movement forwards of the trunk by the alternate advancement of the lower I. - 106

limbs. Suppose a person to be standing erect, with one leg a little in advance of the other; the body, being inclined slightly forwards, is pushed in advance by the extension of the hindmost limb, so that the weight falls more and more upon the advanced leg, which at the same time is shortened by bending the knee and ankle. The heel of the hindmost limb being then raised by the action of the muscles of the calf, the toes press against the ground so as to push the trunk so far in front of the advanced limb as to be no longer safely supported by it; the hindmost limb is then raised from the ground by muscular action, and allowed to swing forward by its own weight, but guided by the muscles, until the toes touch the ground in front of the opposite limb. A step has now been made, and the limbs are in a corresponding but oppositè position from that in which they were when the step commenced: a repetition of the act constitutes another step, and so. the alternate action continues. At one moment in each step both feet touch the ground at the same time, i.e., when the hind foot presses against the earth. The act of running consists in a repetition of the movements of walking performed with so much greater rapidity that the feet never touch the ground at the same moment; the heels also are never brought to the ground. The propulsive action is also greatly increased by the extension of the hip and knee joints, so that a succession of small leaps on to alternate feet takes place. In leaping from the standing position the joints of both lower limbs, previously flexed, are suddenly and simultaneously extended, and the body is projected forwards with a rapid impulse.

Development and Homologies of the Voluntary Muscular System. The voluntary muscles, like the bones and joints with which they are so intimately associated, are developed out of the middle of the three layers-the meso-blast-into which the germinal area or blastoderm of the young embryo is divided. The muscles of the axial skeleton are capable of subdivision into a group situated outside the endo-skeleton, i.e., between it and the integument which muscles have recently been called epi-skeletal and a group lying on the ventral surface of the vertebral bodies and within the rib arches, which have been termed the hæmal or hypo-skeletal muscles. The epi-skeletal muscles, like the vertebræ themselves, are developed within the proto-vertebrae, but it is not known if the hypo-skeletal group have the same origin. In fishes the episkeletal muscles preserve their fundamental arrangement with but little modification. They are disposed in transverse segments or myotomes, which equal in number the vertebræ. These myotomes are separated from each other by bands of fibrous tissue, the intermuscular septa. In man and the higher vertebrates the simple transversely segmented arrangement is to a large extent lost. Traces are preserved, however, in the interspinales and intertransversales muscles, situated in the intervals between the spines and transverse processes of some of the vertebral segments; in the external intercostals and in the recti abdominis muscles, in the last-named of which tendinous bands subdivide the muscle into several transverse segments. More usually, the intermuscular septa either are not formed or disappear, and adjacent myotomes become blended into a continuous mass of muscle. In some instances the fibres of this muscle run longitudinally, and the entire mass subdivides longitudinally into separate and distinct parallel muscles, as is seen in the subdivision of the great erector spine into the sacro-lumbalis, musculus accessorius, cervicalis ascendens, longissimus dorsi, transversalis cervicis, trachelo-mastoid, and spinalis dorsi muscles. In other instances the muscles run obliquely; some on the back of the body pass obliquely from below upwards and outwards, as the splenius and obliquus inferior; others obliquely from below, upwards and inwards, as the complexus, obliquus superior, semispinalis, multifidus and rotatores spina; others again, as the external and internal oblique muscles of the abdomen, extend obliquely from

behind forwards to the ventral mesial line.

Of the hypo-skeletal group of muscles, the internal intercostals display the transverse segmentation. As a rule, however, the muscles of this group extend longitudinally, and form the præ-vertebra, group, named anterior recti, longi colli, and psoæ; though the diaphragm, triangulares sterni, transversi abdominis, and levatores ani, which lie in relation to the inner surfaces of the ribs and visceral cavities, are not longitudinal, but are specially modified in arrangement for functional reasons. The plane of demarcation between the hypoand epi-skeletal groups of muscles, where they form together the

walls of the great visceral chambers, -the thorax and abdomen,— is marked off by the position and course of the intercostal series of spinal nerves. The muscles of the appendicular skeleton are either limited to the limbs (purely appendicular, therefore), or pass from the axial part of the body to the limb (axi-appendicular). The axi-appendicular group are undoubtedly prolongations of the axial system of muscles. They are in the upper limb derived from the epi-skeletal subdivision, and form the trapezius, rhomboid, levator anguli scapula, latissimus dorsi, serratus magnus, greater and smaller pectorals, and subclavius muscles of each superior extremity. In the lower limb they are in part derived from the hypo-skeletal subdivision, and form the psoas and pyriformis; and in part, as the gluteus maximus, from the epi-skeletal subdivision. It is not improbable that the purely appendicular muscles are also prolongations of the axial system, and that as the limbs, in their development from their fundamental bud-like lappets, undergo both a prolonged into them, differentiates both transversely and longitransverse and a longitudinal segmentation, so the muscular mass, tudinally into a motor apparatus, fitted for the performance of the special functions of each extremity.

ANATOMY OF THE TEXTURES OR TISSUES.
Introductory.

Before proceeding to the description of the other organic systems of which the human body is built up, it may be well to enter into the consideration of the minute or microscopic structure of its constituent parts. These parts may primarily be divided into fluids and solids. The fluids are the blood, the lymph, the chyle, the secretions of the various glands, and of the serous and synovial membranes. The solids form the framework of the several organic systems, and assume different appearances in different localities. Sometimes they are arranged in compact solid masses, as in cartilage; at others they are elongated into fine threads or fibres, as in muscle, tendon, nerve; at others they are expanded into thin membranes, as in the fascia or aponeuroses, the serous, synovial, and mucous membranes; at others they are hollowed out into distinct tubes for the conveyance of fluids, as in the blood-vessels, the lymph and chyle vessels, and the ducts of glands. To the solids of the body, whatever their form may be, the general name of Tissues or Textures is applied. Each organic system may be regarded as in the main composed of a tissue or texture peculiar to and characteristic of itself. Thus, the bones are essentially composed of the osseous tissue, the muscles of the muscular tissue, the nervous system of the nervous tissue, fibrous membranes of the fibrous or connective tissue, &c. But though the essential constituent of each organic system is a tissue peculiar to that system, yet in most localities certain other tissues are mingled with that which is to be regarded as the characteristic texture of the part. In a muscle, for example, not only is the muscular tissue present, but mingled with it are connective tissue, nerve tissue, bloodvessels, and lymph-vessels. A gland also not only consists of its proper tissue, the secreting cells, but of more or less connective tissue, nerves, blood and lymph vessels, and gland ducts. Indeed, there are few localities in which, along with the proper tissue of the part, blood and lymph vessels, nerves and connective tissue, are not found; and to a part built up of two or more tissues the name of an Organ is applied. Thus the muscular system consists of the series of organs which we call the muscles, the glandular system of the several organs called glands, and so on. Each tissue and each organ, into the construction of which that tissue enters as the characteristic texture, possesses not only distinctive structural, but also distinctive functional properties. Thus the muscular tissue is characterised by the property of contractility, and the muscles, of which it forms the essential texture, are organs of motion or locomotion; the osseous tissue is characterised by its

hardness and strength, and the bones, of which it forms the essential texture, are organs of protection and support. But the study of the textures embraces an inquiry not only into the special, structural, and functional properties of each tissue and organ-into the special part which each plays in the animal economy-but the consideration of their properties as living structures. It would be out of place in this article to enter into a discussion of the meaning of the term LIFE, or LIVING, or to attempt an analysis LIVING, or to attempt an analysis of the various definitions of the term which have been suggested from time to time by different philosophers, which will naturally find a place in the article PHYSIOLOGY. It will suffice for our present purpose to adopt the old Aristotelian definition, and to speak of Life as the faculties of self-nourishment, self-growth, and self-decay. All the tissues, over and above the special properties which they possess, have the power of growing and of maintaining themselves in full structural perfection and functional activity for a given period of time. After a time they begin to exhibit signs of diminished perfection and activity, they degenerate or decay; ultimately they die, and the entire organism of which they form the constituent parts is resolved by the outrefactive process into more simple forms of matter.

GENERAL CONSIDERATIONS ON CELLS.

The simplest form of organic matter capable of exhibiting the phenomena of life is called Cyto-blastema or Protoplasm. It possesses a viscous or jelly-like consistency. Under the highest powers of the microscope it seems to be homogeneous, or dimly granulated, like a sheet of ground glass. Not only can it assimilate nutriment and increase in size, but it possesses the power of spontaneous movement and contractility. It enters in a very important manner into the structure of the bodies of the lower animals. The elongated processes, or pseudopodia, to which Dujardin applied the name of sarcode, which the Rhizopoda can project from their surface into the surrounding medium, and again withdraw into their substance, consist of protoplasm, and may be cited as furnishing excellent examples of its motive and contractile power. From the recent researches of Haeckel it would appear that protoplasm is capable of forming, without the super- FIG. 25.- Undifferentiated addition of any other structure, inde- cytode mass of protoplasm. pendent organisms, which stand at the lowest grade of organisation, and from their extreme simplicity are named by him Monera. To the group Monera belong the genera Protamoba, Protogenes, and Bathybius. Of these, Bathybius is that which has attracted most attention. It has been regarded as a layer of soft slimy undifferentiated protoplasm covering the bottom of the deep sea, and capable of exhibiting the phenomena of contractility, growth, assimilation of food, and reproduction. Doubts, however, have been expressed regarding the nature of this Bathybius, so that it cannot now be cited as so definite

--P

N

N

body of a higher organism, he has given the general name of a Cytode. Sometimes a cytode is a naked clump of Cytode. soft protoplasm, without a trace of differentiation either on its surface or in its substance, as in the freely-moving Monera; at others the peripheral part of the cytode hardens, and differentiates into a more or less perfect envelope, as in the genera Protomonas and Protomyxa. So far back as 1861, Lionel Beale had described, under the name of germinal matter (Bioplasm), minute living particles of vegetable protoplasm, and in 1863 he demonstrated the presence of extremely minute particles of living matter in the blood. More recently Stricker has also called attention, in the bodies of the higher animals, to minute detached clumps of protoplasm which exhibited the phenomena of life.

As a rule, however, in both vegetable and animal Cell. organisms the specks or clumps of protoplasm assume definite shapes, and show evidence of an internal differentiation. In the midst of a minute clump of this substance a sharply-defined body called a nucleus is found, which differs from the surrounding protoplasm in not being contractile; and sometimes a minute speck, or nucleolus, exists within the nucleus. When a definite clump of protoplasm contains a nucleus in its interior, whether a nucleolus be present or not, it is called a Nucleated Cell. Cells are definite anatomical and physiological units, and exhibit all the phenomena of life. Some of the lowest organisms consist merely of a single cell, others of two or more cells united together, and these are called uni- or multi-cellular organisms. Cells also enter in the most material manner into the constitution of the textures of all the higher forms of plants and animals. Not unfrequently the peripheral part of the protoplasm of the cell differentiates into a distinct investing envelope, technically named a cell wall or cell membrane.

In the earlier periods of investigation into the minute structure of cells it was believed that a cell wall was constantly present, and that each cell was a minute microscopic vesicle or bladder, which in its typical shape was globular or ovoid, but capable of undergoing various modifications both in form and chemical composition. The material enclosed by the cell wall was termed the cell contents, and either in the midst of these contents or in contact with the cell wall was the nucleus, which might or might not contain a nucleolus. Schwann believed that the cell wall was the most active constituent of the cell, i.e., possessed the power not only of producing chemical and physical changes in its own substance and in the cell contents, but of separating materials from the surrounding media,-of secreting them, as it were, into the interior of the cell. In this manner he accounted for the formation in some cells of fat, in others of pigment, in others of the characteristic secretion of glands, and so on.

It was then maintained by John Goodsir that the nucleus was the part of a cell which in all probability was concerned in separating and preparing its characteristic cell contents, and in its nutrition. Martin Barry and Goodsir also contended that the reproduction and multiplication of cells were due to self-division of the nucleus, which was thus the source of successive broods of young cells. They gave to the nucleus, therefore, an importance

could be observed Hence the importance of the cell wall as an essential component of a cell was still further diminished; and Leydig then defined a cell to be a little mass composed of a soft substance enclosing a central nucleus.

But a most important advance in our conceptions of the essential structure of a cell was made when Brücke pointed out that the contents of cells not unfrequently possessed the property of spontaneous movement and contractility, and when Max Schultze determined that the contractile substance termed sarcode, which forms so large a part of the bodies of the lower animals, was analogous and apparently homologous with the contents of young actively-growing animal and vegetable cells, before a differentiation of these contents into special secretions or other materials had taken place. As the term "protoplasm" had been introduced by Von Mohl to express the contents of the vegetable cell, which undergoes changes in the process of growth, it was adopted by the animal histologist; and Max Schultze suggested that a cell should be defined to be a nucleated mass of protoplasm,-a definition which is adopted in this article. Now, as protoplasm, whether it occurs along with a nucleus in the form of a cell, or in independent clumps or cytodes, exhibits not merely the property of contractility, but the power of growing and maintaining itself, it is regarded as the functionally active constituent of the cell. And thus our conceptions as to the part of the cell in which its functional activity resides have passed through three phases. In the first, the cell wall; in the second, the nucleus; in the third, the protoplasm cell contents, or cell substance, has been regarded as the active constituent, not only as regards its nutrition, but the reproduction of young cells. But though the protoplasm can of itself perform these offices, yet there can be no doubt, as Barry and Goodsir were the first to show, that the nucleus of the cell plays a part not unfrequently in the multiplication of cells by self-division.

N..

One of the most characteristic cells is the mammalian ovum. In it a cell wall exists, known as the zona pellucida or vitelline membrane; within this envelope is the granular yelk or cell contents, in the midst of which is imbedded the nucleus or germinal vesicle, which in its turn contains the nucleolus

or germinal spot. The granules of FIG. 27.-Ovum of a sheep. the yelk are a special metamorphosis of the protoplasm cell substance.

W, cell wall or zona pellucida; P, protoplasm of yelk: N, nucleus, or germinal vesicle; M, nucleolus, or germinal spot.

Schwann made the important generalisation that the tissues of the animal body are composed of cells, or of materials derived from cells, "that there is one universal principle of development for the elementary part of organisms, however different, and that this principle is the formation of cells." The ovum is the primordial or fundamental cell, or germ-cell, from which, after being fertilised by the male sperm, the tissues and organs of the animal body are derived. Within the fertilised ovum multiplication of cells takes place with great rapidity. It is as yet an unsettled question how far the original nucleus of the ovum participates in this process of multiplication; but there can be no doubt that the protoplasm cell contents divide, first into two, then four, then eight, then sixteen segments, and so on. Each of these segments of protoplasm contains a nucleus-is, in short, a nucleated cell, and the protoplasm of these cells exhibits the property of contractility. The ovum or germ-cell is therefore the immediate parent of all the new cells which are formed within it, and mediately it is the parent of all the cells which, in the subsequent processes of development and growth,

are descended from those produced by tne segmentation of the yelk. The process of development of young cells within a parent cell, whether it occurs in the ovum or in a cell derived by descent from the ovum, is called the endogenous reproduction of cells. But cells may multiply by a process of fission-i.e., a constriction, gradually deepening, may take place in a cell until it is subdivided into two; the nucleus at the same time participating in the constriction and subdivision. A third mode of multiplication of cells is by budding: little clumps of protoplasm bud out from the protoplasm of the parent cell, become detached, and assume an independent vitality. If a nucleus differentiates in the interior of such a clump, it becomes a cell; if it remains as a mere clump of protoplasm, it is a cytode.

These various methods of multiplication are all confirmatory of Schwann's generalisation of the descent or derivation of cells from pre-existing cells. But as the nucleated cell, either with or without a cell wall, is not, in the present state of science, regarded as the simplest and most elementary unit capable of exhibiting vital phenomena, and as these phenomena can be displayed by individual clumps of protoplasm, without the presence of a nucleus, some modification of the doctrine, as regards the formation of the tissues from nucleated cells, seems to be necessary. For, although there can be no doubt that all the tissues are mediately derived from the ovum or fundamental cell, and that most of the tissues are derived directly from nucleated cells, yet there is reason to think that a differentiation of a cytode clump of protoplasm into tissue may take place, so that the direct formation of such a tissue would be, not from a nucleated cell, but from the more simple cytode. Hence a more comprehensive generalisation, to which observers have gradually been led from the consideration of numerous facts, has now been arrived at,-that the tissues and organs of the body, whatever may be their form and composition, are formed of protoplasm, or produced by its differentiation; and that the protoplasm itself is derived by descent from the protoplasm substance of the primordial germ-cell. Some, indeed, have contended that protoplasm, cells, and their derivatives can arise by a process of precipitation or aggregation of minute particles or molecules in an organic infusion, and that living matter may be thus spontaneously generated. But the evidence which has been advanced in support of this hypothesis is by no means satisfactory or conclusive, whilst the correctness of the theory of the direct descent of protoplasm from pre-existing living protoplasm is supported by thousands of observations made by the most competent inquirers.

In the process of conversion of protoplasm into the several tissues, there takes place a differentiation of form and structure (i.e., a morphological differentiation), and of composition (ie., a chemical differentiation), as the result of which a physiological differentiation is occasioned, whereby tissues and organs are adapted to the performance of special functions. Hence arise the several forms of tissue which occur in the human body and in the higher animals. Many of the tissues consist exclusively of cells which present in different parts of the body characteristic modifications in external configuration, in composition, and in properties, as may be seen in the fatty tissue, pigmentary tissue, and epithelium. Other tissues, again, consist partly of cells, and partly of an intermediate material which separates the constituent cells from each other. Here also the cells present various modifications; and the intermediate material, termed the matrix or intercellular substance, varies in structure, in composition, and in properties in the different textures, as is seen in the connective, cartilaginous, osseous and muscular tissues