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The Trochlearis or fourth, the smallest cranial nerve, lies at the outer side of the crus cerebri. When traced backwards to its origin it is seen to sink into the valve of Vieussens, where its fibres divide into three roots: one decussates across the valve with a root of the corresponding nerve on the opposite side; another passes backwards to the locus ceeruleus; the third sinks into the corpora quadrigemina and reaches the Sylvian group of nerve cells, from which the third nerve also arises. The fourth nerve runs forward in the outer wall of the cavernous sinus, enters the orbit through the sphenoidal fissure, and ends in the superior oblique muscle. It also communicates with the cavernous plexus of the sympathetic.
The Abducent or sixth nerve springs out of the groove between the lower border of the pons and the anterior pyramid of the medulla oblongata. Its roots sink deeply into the pons, and arise from a nucleus of grey matter at the floor of the fourth ventricle, common to it and the portio dura. The sixth nerve runs forward in the inner wall of the cavernous sinus, enters the orbit through the sphenoidal fissure, and ends in the external rectus muscle. It communicatcs with the carotid plexus of the sympathetic.
The I‘ortio dura or motor facial portion of the seventh nerve springs out of the groove between the lower border of the pons and the restiform body. Its roots sink deeply into the pons, and whilst some of its fibres arise from a grey nucleus, at the floor of the fourth ventricle, common to it and the sixth nerve, others ascend from a nucleus which, according to Meynert, lies just on the outer side of the superior olivary body, and others again decussate across the median raphe of the pons. An accessory portion, called portio intermedia, which is said to arise from the lateral columns of the cord, joins the portio dura. The portio dura enters the internal auditory meatus in the petrous-temporal bone along with the auditory nerve ,' but at the bottom of the meatus it leaves that nerve and enters the aqueduct of Fallopius along which it is conducted through the bone to emerge at the stylo-mastoid foramen. \Vhcn in the aqueduct it forms a knee-shaped bend, and expands into a small ganglion, which is joined by the great, small, and external petrosal nerves, and through the external petrosal it communicates with the sympathetic. The portio dura gives off—a, a minute branch to the stapedius muscle ; b, the chorda tympani, which, entering the tympanum, passes across that cavity, emerges through the Glaserian fissure, and joins the lingual branch of the fifth nerve, which it accompanies as far as the submaxillary ganglion; it gives a branch to the ganglion, and one to the lingualis muscle. After the portio dura has passed through the stylo-mastoid foramen it gives ofl—c, the posterior auricular branch to the occipital belly of the occipitofrontalis and to the retrahens aurem muscle, and d, the digastric branch to the posterior belly of the digastric and stylo-hyoid muscles ; and then runs forwards through the parotid gland to the face, where it breaks up into numerous (e) facial branches to supply the facial muscles of expression and the buccinator muscle. The facial is also the secretory nerve for the salivary glands. Through the chorda tympani it influences the secretion of the submaxillary and sublingual glands, and through the connection between its lesser petrosal nerve and the auriculo-temporal in the otic ganglion it influences the parotid gland.
The Spinal Accessory is the lowest division of the eighth nerve. It springs out of the side of the medulla oblongata, and from the lateral column of the cervical part of the spinal cord as low as the fifth cervical nerve: its roots arise from the intermedio-lateral group of nerve cells in the cord, and from a nucleus of grey matter in the floor of the fourth ventricle. The spinal fibres of origin enter the skull through the foramen maa'um, join the fibres from the
medulla, and leave the cranial cavity through the jugular foramen. This nerve, purely motor in function, is subdivided into two parts, an internal and an external. The external passes obliquely outwards across the side of the neck, pierces the sterno-mastoid, and ends in the trapezius, both of which muscles it supplies. The internal joins the pneumogastric nerve, of which it forms the motor or accessory root, and is distributed along with it.
The Hgpoglossal or ninth nerve springs out of the groove between the anterior pyramid and olivary body of the medulla oblongata, in series with the anterior roots of the spinal nerves. Its roots pass through the medulla to the floor of the fourth ventricle, to arise from the nerve cells in two nuclei of grey matter situated close to the median furrow. This grey matter is in series with the anterior cornua in the spinal cord. The nerve passes out of the skull through the anterior condyloid foramen, and arches across the side of the neck to the tongue, to end in glossai branches for the supply of the intrinsic and extrinsic muscles of the tongue. It also gives off—a, the descendcnsnoni branch, which, after been joined by the commum'cantcs noni from the cervical plexus, supplies the omo-hyoid, sterno-hyoid, and sterno-thyroid muscles ; b, the thyro-hymki branch to the thyro-hyoid muscle ; c, the genio-hyoid branch to the genio-hyoid muscle. It communicates in the neck with the sympathetic, vagus, lingual branch of the fifth, and cervical plexus.
The group of mixed nerves will now be considered.
The Trifaeial or fifth is the largest cranial nerve.
The smaller or motor root arises from the nerve cells of a nucleus of grey matter situated in the back of the pons, near the floor of the upper part of the fourth ventricle. The larger or sensory root has, according to Meynert, a complex origin—a, from a nucleus of grey matter in the pons to the outer side of the origin of the motor root; 6, by descending fibres which arise from nerve cells in the substance of the corpora quadrigemina, from the grey matter of the locus caerulens, and from the longitudinal
fibres of the pons ; c, by ascending fibres which apparently
arise from the grey tubercle of Rolando; cl, probably by fibres which traverse and embrace the superior peduncle of the cerebellum. As the large sensory root of the fifth lies on the petrous bone it expands into the Gasserian ganglion, which resembles in structure the ganglion on the posterior root of a. spinal nerve. From this ganglion three large branches arise, named respectively the lst, 2d, and 3d divisions of the ganglion.
The lst or Ophthalmic division is the upper sensory nerve of the face, and divides into three branches, which pass out of the cranial cavity through the sphenoidal fissure. By its lachrgmal branch it supplies the lachrymal gland, and the outer part of the skin and conjunctiva of the upper eyelid 3 by its frontal branch, the inner part of the skin and conjunctiva of the upper lid, and the skin of the forehead ;_ by its oculo-nasal branch, it gives long ciliary nerves to the eyeball, and a nasal nerve to the mucous membrane of the nose, and the skin of the side of the nose. From the oculo-nasal nerve arises the long or sensory root of the ciliary ganglion, which lies in the cavity of the orbit, and which receives also a motor root from the third nerve, and a root from the sympathetic. This ganglion gives origin to the short ciliarg nerves for the eyeball.
The 2d or Superior Maxillary division is the sensor],' nerve for the middle part of the face. It leaves the skull by the foramen rotundum, passes across the spheno-maxillm y fissure, then lies in the canal in the floor of the orbit, from which it emerges on the face through the infra-orbital foramen as the infra~orbital nerve. It gives off a small orbital branch to a small part of the skin of the temple.
T. — In
It Mixed springs by two distinct roots out of the side of the pons. cranial and that ow er the cheek bone; dental branches to the teeth in the upper jaw; palpcbral branches to the skin and conjunctiva of the lower eye-lid; nasal branches to the skin and mucous membrane of the nose ; labial branches to the skin and mucous membrane of the upper lip. It also gives ofi', when in the spheno-maxillary fossa, sphenopalaline branches, which form the sensory root of _the spheno-palatine or Meckel’s ganglion. This ganglion receives a motor root through the great petrosal nerve from the kneeshaped bend of the portio dura, and a' sympathetic root from the carotid plexus, which runs along with the great petrosal, and forms with it the vidian nerve. The ganglion gives origin to—a, an orbital branch, which supphes a layer of non-striped muscular fibres, described by H. Miiller and Turner as developed in connection with the periosteum of the orbit, where it covers the spheno-maxillary fissure; 6, upper nasal and naso-palatine branches to the mucous membrane of the nose and hard palate; c, descend ing palatinc branches to the mucous membrane of the hard and soft palate; d, pterygogaalatine to the mucous membrane of the upper part of the pharnyx.
The 3d or Inferior Maxillary division passes out of the skull through the foramen ovale, and as it does so is joined by the motor root of the 5th. By the junction a mixed nerve is formed, which is the sensory nerve for the lower part of the face, and the skin of the temple, and the motor nerve for the muscles of mastication. Immediately after passing through the foramen this nerve divides into a small and large division, in each of which motor and sensory fibres are found. The small division supplies motor maslz'catory branches to the masseter, temporal, external, and internal pterygoid muscles; but further it gives off a long bnccal branch, which, though often described as the motor nerve for the buccinator muscle, is really a sensory nerve for the skin and mucous membrane of the cheek. The sensory nature of this nerve is proved, not only by physiological and pathological experiments, but by tracing its fibres through the buccinator muscle to the mucous membrane. Turner has also recorded two cases in which the long buccal nerve arose as a branch of the sensory superior maxillary nerve. The large division separates into three branches—a, auriculo-temporal, which ascends to supply the parotid gland, the skin of the auricle, external mcatus, and temple, and the temporo-maxillary joint ; b, inferior dental, which enters the dental canal in the lower jaw, and supplies the lower set of teeth and the skin and mucous membrane of the lower lip ; it also gives off a mylo-hyoid branch to the mylo-hyoid and anterior belly of the digastric muscle; 0, lingual or gustatory, which runs forward along the side of the tongue to end in the filiform and fungiform papillae of its mucous membrane. The lingual branches are sensory nerves of touch, though some physiologists believe that they are also nerves of taste. Connected with the branches of the inferior maxillary division are two small ganglia, which, like the ciliary and spheno-palatine ganglia, are of a greyish colour, contain nerve cells, and receive roots from motor, sensory, and sympathetic nerves. The submaxillary ganglion lies under cover of the mylo-hyoid muscle, and receives a root from the motor chorda tympani nerve, a root from the sensory lingual, and a sympathetic root. It gives branches to the sub-maxillary and sublingual salivary glands. The otic ganglion lies close to the Eustachian tube, and receives a root from the muscular nerve to the internal pterygoid, a root from the sensory auriculo-temporal, and a sympathetic root. It also receives the small petrosal nerve, by which it is connected to the knee-shaped bend of the portio dura and to the glossopharyngeal nerve. It supplies the tensor tympani and tensor palati muscles. The branches of the three divisions of the fifth cranial nerve, which pass to the skin of
the temple, forehead, and face, freely communicate with the branches of the portio dura, which supply the muscles situated in those regions.
The Glosso-pharyngeal or uppermost division of the eighth nerve springs out of the side of the medulla oblongata between the olivary and rcstiform bodies; its roots arise from two small masses or nuclei of grey matter inthe floor of the 4th ventricle. The nerve passes out of the skull through the jugular foramen, where it possesses two small ganglia, named jugular and petrous. It then passes across the side of the neck and gives off carotid branches, which run along the internal carotid artery; pharyngeal branches to the mucous membrane of the pharynx; tonsilitic branches to the tonsil and soft palate; glossal branches to the base of the tongue and the circumvallats papillm, which branches are unquestionably nerves of the special sense of taste; muscular branches to the stylepharyngeus and perhaps the constrictor muscles. Through the jugular and petrous ganglia the nerve communicates with the vagus and sympathetic. The petrous ganglion gives off the tympanic branch or nerve of Jacobson, which enters the tympanic cavity, supplies its mucous membrane, and gives off three communicating branches—one to the sympathetic ,' a second to the great petrosal, and through it to the knee-shaped bend of the facial; a third to the small petrosal, and through it to the otic ganglion.
The Pneumogastric or Vagus is the middle subdivision of the eighth cranial nerve. It springs out of the side of the medulla oblongata, between the olivary and restil'orm bodies; its roots arise from a nucleus of grey matterin the floor of the 4th ventricle, which nucleus, along with those for the glosso-pharyngeal nerve, is in series with the posterior cornu of grey matter in the spinal cord. It goes through the jugular foramen, is joined by the inner division of the spinal accessory which is its motor root, then passes down the side of the neck, enters the thorax, reaches the outer wall of the (esophagus, accompanies that tube through the diaphragm, and terminates in the wall of the stomach. The left nerve lies on a plane anteriorto the right : it crosses in front of the arch of the aorta, and tributed to the anterior wall of the stomach, whilst the right nerve supplies the posterior wall. Each nerve possessei high in the neck two enlargements, named upper and lower ganglia. The branches of the vagus are numerous and important. The upper ganglion gives origin to the auricular branch, which traversing a small canal in the petrous temporal bone, is distributed to the skin of the back of the cuticle. The lower ganglion gives origin to—a, the pharyngeal branch, which forms a plexus with the glosso-pharyngeal and sympathetic nerves, from which the muscles of the pharynx are supplied; 6, the superior laryngeal, which divides into an external branch to supply the crico-thyroid muscle, and an internal, which pierces the thyro-hyoid membrane, and supplies the mucous lining of the larynx and the mucous covering of the epiglottis. The trunk of the nerve gives origin to—a, the recurrent laryngeal branch, which on the right side turns round the subclavian artery, and on the left round the arch of the aorta, and ascends to the larynx to supply its intrinsic muscles except the crico-thyroid; 6, cardiac branches, which arise from the nerve partly in the neck and partly in the chest, and join the great cardiac plexus for the heart ; c, pulmonary branches, which arise in the chest, pass into the substance of the lungs, and form along with the sympathetic an anterior plexus in front of, and a posterior plexus behind the root of the lung; (1, a’sophageal branches, which supply the coats of the oesophagus; e, gastric branches, which supply the coals of the stomach, and give important offshoots to the great solar plexus ofnthe sympathetic situated at the pit of the stomach.
Dnscmr'rivn ANATOMY or run Sms'rns'rro Nsnvous Svsrinr.
lympalhe- The Sympathetic Nervous System consists of a pair of 11C nervous gangliated cords, situated one on each side of the spinal
‘Yhtcm' column; of three great gangliated prevertcbral plexuses situated in the thoracic and abdominal cavities ,- of numerous smaller ganglia lying more especially in relation with the thoracic and abdominal viscera; of multitudes of fine distributory nerves.
‘Ynngliated Each Gangliated Cord of the sympathetic extends along
3:164:11: the side of the spine from the base of the skull to the
coccyx. In the neck it lies in front of the transverse processes of the vertebrae ,' in the thorax, in front of the heads of the ribs 3 in the abdomen, on the sides of the vertebral bodies; and as it descends in front of the sacrum it approaches its fellow, so that in front of the coccyx the two are united in a single ganglion, the ganglion impar (Fig. 66, 0). Each cord consists of a number of ganglia united into a continuous cord by intermediate nerves. As a rule, the ganglia equal in number the vertebrae of the region. Thus, in the sacral region there are five ganglia, in the lumbar five, and in the thorax twelve ; but in the neck there are only three, named superior, middle, and inferior ; of these the superior is very large, and represents without doubt several smaller ganglia. From the superior cervical ganglion the cord is prolonged upwards by an ascending or cranial ofi‘shoot through the carotid canal into the cranial cavity, and forms a plexus around the internal carotid artery, both in the carotid canal, named the carotid plexus, and in the inner wall of the cavernous sinus, named the cavernous plexus. Through branches derived either directly or indirectly from these plexuses the sympathetic roots for the ciliary and spheno—palatine ganglia, described in connection with the fifth nerve, are derived.
From the gangliated cord and its ascending or cranial prolongation a communicating and a distributory series of branches are derived.
By the Communicating branches this portion of the sympathetic is connected with most of the cranial and with the anterior divisions of all the spinal nerves, so as to bring the cerebro-spinal and sympathetic systems into close anatomical and physiological relation with each other. It is important also to observe that each communicating branch contains not only non-mcdullated nerve fibres from the sympathetic system to the cerebro-spinal nerves, but medullated fibres from the cerebro-spinal to the sympathetic, so that a double interchange takes place between the two systems. The cranial prolongation of the sympathetic and the superior cervical ganglion communicate with the 3d and 4th nerves, the Gasserian ganglion of the 5th, the 6th, the portio dura of the 7th, the glosso-pharyngeal and pneumogastric of the 8th, and the 9th,cranial nerves, and with the anterior divisions of the four upper cervical spinal nerves. The middle cervical ganglion communicates with the 5th and 6th cervical nerves, the inferior cervical ganglion with the 7 th and 8th cervical nerves, the twelve thoracic ganglia with the series bf intercostal nerves, the five lumbar ganglia with the series of lumbar spinal nerves, the sacral and coccygeal ganglia with the sacral nerves and the coccygeal nerve.
The Distributorg branches of the gangliated cord are as follows :——a, PIzarg/ngeal branches from the superior cervical ganglion, which join the pharyngeal branches of the glossopharyngeal and pneumogastric nerves, to form the pharyngeal plexus, which supplies the muscles and mucous membrane of the pharynx. b, Articular branches from the upper thoracic and the lumbar ganglia to the articulations between the adjacent vertebrae. 0, Pulmonary branches from the 3d or 4th thoracic ganglia, which join the posterior
pulmonary plexus. d, Vaao-motor branches ornenn' molla, which supply the muscular coat of the arteries : those which arise from the cranial prolongation of the superior cervical ganglion supply the internal carotid artery and its branches to the brain and eyeball : those which arise from the superior cervical ganglia itself supply the external carotid artery and its branches; from the branch accompanying the facial artery the submaxilliary ganglion derives its sympathetic root 3 from that accompanying the middle meningeal artery the otic ganglion derives its sympathetic root : the vaso-motor nerves which arise from the middle cervical ganglion supply the inferior thyroid artery, and pass to the thyroid gland: the vase-motor branches of the inferior cervical ganglion supply the vertebral and basilar arteries and their several branches, which pass to the spinal cord and the binder part of the encephalon. Vaso-motor nerves also arise from the thoracic ganglia, which pass to the thoracic aorta, from the lumbar ganglia to the abdominal aorta, and from the sacral ganglia to the middle sacral artery; the ganglion impar gives branches to a peculiar vascular structure, named the coccggeal body, developed in connection with the end of the middle sacral artery ,- a body of similar structure, called intercarotic body, situated in the angle of bifurcation of the common carotid artery, receives branches from the superior cervical ganglion. e, Cardiac branches fromthe superior, middle, and inferior cervical and the lst thoracic ganglia, which pass into the thorax to join the prevertebral cardiac plexus. f, Splanclznic branches as follows: great splanclmic nerve, by the union of branches from the thoracic ganglia, the 3d to the 10th inclusive; it pierces tho crus of the diaphragm, and passes to the prevertebral solar plexus; small splanclmic nerve, also to the solar plexus from the 10th or llth thoracic ganglia ; smallest splanchm'c nerve, from the 12th thoracic ganglion to the renal plexus. g, H ypogastric branches, from the lumbar and sacral ganglia to the prevertebral hypogastric plexus.
The Prevertebral Cardiac plexus (Pl. XVII. c) is situated Gangliated. at the base of the heart, and is divided into a superficial Prev"part, which lies in the concavity of the arch of the aorta, files and a deep part between the aorta and trachea. It receives p ' the cardiac branches of the pneumogastric and the cervical ganglia of the sympathetic. It contains collections of nerve cells and a dense plexiform arrangement of nerve fibres. It gives oil‘ branches to the heart, which wind around the surface of that organ and penetrate its muscular substance: on these branches minute ganglia are found which regulate its rhythmical movements. Through these branches and the cardiac plexus the heart is brought into connection with both the cerebro-spinal and sympathetic systems of nerves. The sympathetic apparently regulates its contraction, for when this nerve is stimulated the action of the heart is accelerated The pneumogastric again exercises an inhibitory or restraining influence on the contractions of the organ, for when this nerve is irritated the activity of contraction is diminished, but when divided it is greatly increased. The cardiac plexus also sends ofl'sets to the anterior and posterior pulmonary plexuses for the supply of nerves to the lungs.
The Prevertebral Solar or Epigastric plea-us is situated at the pit of the stomach around the cmliac axis, a branch of the abdominal aorta. It receives the great and small splanchnic nerves from the thoracic ganglia of the sympathetic, and some of the terminal branches of the pneumogastric nerve. It contains large collections of nerve cells, which form the two semilunar ganglia, and a dense plexiform arrangement of nerve fibres. It gives origin, either directly or indirectly, to numerous plexiform branches, which accompany, and are named after, the abdominal aorta and its various branches given off to the walls and viscera of the abdomen proper. In this manner, not only
do the arteries which supply the abdominal viscera receive their vaso-motor nerves, but the muscular and mucous coats of the stomach, intestines, gall bladder, bile ducts, ureters, and seminal ducts, and the glandular structures of the liver, pancreas, kidneys, spleen, and supra-renal capsules. It is important also to observe that these plexuses of distribution not unfrequently contain small ganglia, and the branches which supply the muscular coat of the stomach and intestines have minute microscopic ganglia, with stellate nerve cells lying amidst them. The distribution of the pneumogastric nerve to the stomach, and its connection with the solar plexus, enables that nerve to stimulate its peristaltic contraction, and, according to some experimenters, that of the small intestine also; but the precise action of the sympathetic on these organs is still a disputed question.
The Prevertebral Hypogastric plexus is situated in front of the last lumbar vertebra. It receives branches from the lumbar ganglia of the sympathetic, and from the plexus surrounding the abdominal aorta. It divides into two parts, which lie one on each side of the rectum, and are called the pelvic plexuses ,' these plexuses are joined by branches from the sacral ganglia of the sympathetic, and from the 3d and 4th sacral spinal nerves, and contain small gangliform collections of nerve cells. From the pelvic plexuses numerous plexiform nerves arise, which accompany the internal iliac artery and its branches to the walls and viscera of the pelvis, and are named after them. These nerves not only supply the vase-motor nerves for these blood-vessels, but also the muscular coat and mucous membrane of the bladder, rectum, and urethra, besides the prostate gland in the male, and the uterus and vagina, and in part the ovary, in the female; in connection with their distribution to these viscera, minute ganglia are found lying amidst the nerves, the nerve cells in which act undoubtedly as centres of reinforcement for the origin of additional. nerve fibres.
From the distribution of the branches of the gangliated cord of the sympathetic, and of the gangliated prevertebral plexuses, it will be seen that this nerve is especially related to the blood-vessels and to the viscera_contained within the great cavities of the body. As the cerebro-spinal sys— tern is engaged in the supply of nerves to the voluntary muscles, the sympathetic is the medium of supply for the involuntary muscular apparatus, both in the coats of the vessels and in the walls of the hollow viscera But though the vase-motor nerves branch from the sympathetic ganglia, it must not be supposed that they have no connection with the cerebro-spinal system. The communicating branches between the sympathetic ganglia and the anterior divisions of the spinal nerves establish a connection between them and the ccrebro-spinal nervous axis. By recent experiments, the tract of transmission of the vasomotor fibrcs has been traced along with the anterior roots of the spinal nerves, through the lateral columns of the cord to the medulla oblongata, in which the vase-motor nerve centre lies a little to one side of the mesial plane, above the calamus scriptorius. In the distribution of the sympathetic to the glandular viscera, not only is it important to attend to their terminations in the muscular coat of the blood-vessels of the glands, but the termination of the nerves in connection with the secreting cells themselves must be taken into consideration. The com— munications between the cercbro-spinal and sympathetic systems, not only through the spinal nerves, but also through the pneumogastric, are to be kept in mind in connection with the effects produced by varying mental conditions on the secretions of the glands.
ORGANS or SENSE.
The organs of sense are the organs through the interme diation of which the mind becomes cognisant of the appearance and properties of the various objects in the external world. These organs are severally named nose, eye, ear, tongue, and skin. For the excitation and perception of a sensation three sets of structures are necessary: a, a peripheral end-organ ; b, a sensory nerve ; c, a central organ. The peripheral end~organ is the part of the apparatus to which the stimulus necessary for the production of the sensation is applied. This stimulus causes nervous impulses to be propagated from the end-organ along the fibres of the sensory nerve to the central organ, in which that nerve terminates at its central extremity. These nervous impulses occasion molecular changes in the nerve cells of the brain, and the mind becomes conscious of a sensation. The shape and construction of each organ of sense is adapted to the application of the stimulus required for the production of the particular sensation to which the organ is subservient. Each organ of sense possesses its own characteristic form of end-organ. The touch corpuscles of the skin, the end bulbs found in several mucous membranes, and the Pacinian corpuscles, are the end-organs occurring in their several localities; they have the peripheral ends of the sensory nerves terminating in their substance, and the axial cylinder of the nerve fibre ends in their interior. The rods and cones of the retina, the rods of Corti in the cochlea, the olfactory cells of the nose, and the gustatory bodies in the tongue, are the end-organs belonging to their several organs of sense; the sensory nerve fibres which terminate in relation with them have not yet, however, been traced into actual continuity with their substance. A stimulus, whatever may be its nature, applied to any organ of sense can excite only that kind of sensation for the production of which the organ is subservient. Thus a stimulus applied to the eye, whether it be the natural stimulus of the waves of light, the mechanical stimulus of a blow, or an electric stimulus, can only excite the sensation of light. Stimuli applied to the car can only excite the sensation of sound, and in like manner with the other senses. In studying the anatomy of the organs of sense the arrangement of numerous accessory structures, which assist either in conducting stimuli or in modifiying their efl‘ects, the arrangement and structure of the peripheral end~organs, and the origin, course, and distribution of the sensory nerves, will have to be considered.
The Nose or organ of smell is a large cavity situated in Nose.
the face, between the orbits, above the mouth, and below the cribriform plate of the ethmoid bone. It communicates by the anterior nares, or nostrils, with the external atmosphere, by the posterior nares with the pharynx, and through it with the larynx, trachea, and lungs. It is the proper entrance to the respiratory passage, is accessory to the production of the voice, aids in the sense of taste, and forms one of the most important features of the face. It is subdivided into a right and a left chamber by a vertical mesial partition, the septum nasi, so that the nose is a double organ in the same sense as the eyes or ears are double. The walls of the cavity of the nose are formed partly of bone and partly of cartilage. The osseous walls are referred to on page 826. The eartilsges form the point, the alzn, and a part of the septum nasi. The mesial or septal cartilage is triangular in shape, and fits into the interval between the vomer, the mesial plate of the ethmoid, and the nasal spine of the superior maxilla. Anteriorly and inferiorly its border is free, projects on to the face, and forms the columna of the nose. The lateral cartilage: form the tip and she. On each side is an upper lateral cartilage attached by its