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and arch. The lingual branches are regarded as the main stem (post-trematic), the pharyngeal branches as subordinate branches; the tympanic branch being the præ-branchial or præ-trematic branch for the anterior margin of the third gill-cleft.
The vagus nerve is generally regarded as representing the fusion of all the branchial nerves behind the glossopharyngeal. Its efferent fibres are in series with those of the
glossopharyngeal above and the accessory nerve below, and belong to the lateral series of His. Its afferent fibres, like those of the glossopharyngeal, represent two elements. The ganglion nodosum has possible connexions with epibranchial sense-organs—the rest of the nerve representing the fused branchial nerves of fishes. The superior laryngeal nerve is looked upon as the branchial nerve of the fourth, and the recurrent nerve as the branchial nerve of the fifth arch. While the relation of the nerve to the hinder gill-arches and clefts makes it possible to understand the innervation by the vagus of the heart and lungs, no satisfactory explanation is forthcoming of the passage of the nerve into the abdomen, and its distribution to the stomach and other organs below the diaphragm.
FIG. 666.—SCHEME TO ILLUSTRATE THE DISPOSITION OF THE MYOTOMES IN THE EMBRYO IN RELATION
TO THE HEAD, TRUNK, AND LIMBS. A, B, C, First three cephalic myotomes; N, 1, 2, 3, 4, Last persisting cephalic myotomes ; C, T, L, S,
Co, The myotomes of the cervical, thoracic, lumbar, sacral, and caudal regions ; I., II., III., IV., V., VI., VII., VIII., IX., X., XI., XII., refer to the cerebral nerves, and the structures with which they may be embryologically associated.
The accessory nerve consists of two parts. The internal ramus (accessory portion) of the nerve consists of efferent fibres for the branchial region, in series with the lateral motor roots of the glossopharyngeal and vagus nerves. The external ramus (spinal portion) of the nerve is also composed of efferent fibres, and represents the only lateral motor elements arising from the spinal medulla.
Olfactory Nerve.—There is complete uncertainty regarding the morphology of this nerve. It consists of three elements : (1) the olfactory buló, derived from the cerebral hemisphere, solid in man, but a hollow cerebral diverticulum in certain animals, and forming the rhinencephalon ; (2) the olfactory ganglion, with its central and peripheral processes, derived from the ectoderm ; (3) the nasal pit. Attention has been specially fixed on the olfactory ganglion, which has been compared to (1) a spinal ganglion, derived from the anterior end of the medullary groove ; and to (2) a lateral line sense-organ.
The optic nerve also presents an insoluble problem in regard to its morphological position in the series of cerebral nerves. The optic stalk and optic cup have been regarded as a highly modified spinal ganglion ; but there is insuperable difficulty in accepting this view. The peripheral processes do not become connected with either ectodermal or mesodermal structures, but become the tissue of the retina ; while the central processes, growing backwards, envelop the optic stalk, and obtain connexions with the brain. The retina must be regarded as a highly modified nerve-layer, morphologically in series with the wall of the fore-brain ; and the ectodermal structure of superficial origin comparable to the olfactory ganglion or the auditory vesicle is the lens (which may possibly be homologous with a lateral line sense-organ). The optic nerve, optic chiasma, and optic tract are then to be looked upon as cerebral commissures, and not as nerves in the ordinary sense.
The simplest and most primitive condition of the head, in relation to the morphology of the cerebral nerves, is found before the formation of the gill-clefts, when the salient features are
Fig. 667.—THE EMBRYOLOGICAL ARRANGEMENT OF THE CEREBRAL NERVES. (Modified from Mall.)
a tubular and simple brain, and a series of superficial invaginations which pass from the surface inwards to become connected with outgrowths corresponding to them from the primitive brain. On each side of the head three hollow invaginations occur:-(1) The nasal pit bearing the olfactory epithelium becomes connected by the olfactory ganglion with the rhinencephalon, an outgrowth from the fore-brain, and so forms the basis of an olfactory organ and nerve ; (2) a similar invagination produces the lens, connected with a protrusion of the optic vesicle from the forebrain, by which the basis of the eye and the optic nerve is formed ; (3) behind the oral cavity a third 'invagination forms the auditory vesicle, which is connected with the solid extension from the hind-brain of the acoustic ganglia, to form the essentials of the organ of hearing and acoustic nerve.
The trigeminal nerve is essentially the nerve of the buccal cavity and the subordinate cavities, nasal and oral, derived from it. The branchial arches and clefts are secondary structures, and their nerves are—(1) the trigeminal, for the first (mandibular) arch and the cleft in front of it; (2) the facial, for the second (hyoid) arch and cleft; (3) the glossopharyngeal, for the third (thyreo-hyoid) arch and cleft; and (4) the vagus, for the succeeding arches and clefts. The cerebral part of the accessory nerve is inseparable from the motor portion of the vago-glossopharyngeal nerves; the spinal part is beyond the series of the cerebral nerves.
Lastly, there are certain truly segmental nerve elements, motor fibres which, remaining associated with certain persistent cephalic myotomes, give rise to the oculo-motor, trochlear, abducent, and hypoglossal nerves.
NOTE.—Since 1913 an additional pair of cerebral nerves, the nervi terminales, has been known in man. The nerves were discovered in 1894 in protopterus, and since then they hare been demonstrated in all groups of vertebrates. In man each nervus terminalis is a very small ganglionated nerve which is attached to the inferior surface of the frontal portion of the brain in the region of the olfactory trigone. In the intracranial part of its course it lies medial to the olfactory tract and bulb and its peripheral filaments accompany the filaments of the olfactory nerve. The functions of the nervi terminales, the course of their fibres, and their associations in the substance of the brain are not known.
ORGANA SENSUUM ET INTEGU
By ROBERT HOWDEN, M.B., F.R.S.E.
The organs of the senses are derived from cells of the ectoderm and constitute the apparatus by which man is made acquainted with his surroundings.
Every sense organ consists of three parts :-(a) a peripheral or receptive portion, capable of responding to external stimuli, (6) an intermediate or conductive part, along which the impulses are conveyed, and (c) a central or perceptive portion, where the impulses are collected and transformed into sensations. The intermediate and central parts have been described in the section on the Nervous System; the peripheral parts form the subject matter of this chapter, and may be grouped under two headings:-(a) those connected with the special senses of smell, sight, hearing, and taste, and located in the nose, eye, ear, and mouth, respectively; and (6) those of general sensations (pressure, heat, cold, pain, etc.), which are widely distributed throughout the body.
The nose is the peripheral olfactory organ and consists of the nasus externus, which projects from the face, and the cavum nasi, which is divided by a vertical septum into right and left cavities.
Nasus Externus.— The external nose forms a more or less triangular pyramid, of which the upper angle is termed the root, and is usually separated from the forehead by a depression, while its base, directed downwards, is perforated by the nares or nostrils. Its free angle is named the apex; and the anterior border, joining root and apex, is termed the dorsum; the upper part of the dorsum is supported by the nasal bones, and is named the bridge. Each side of the nose forms an open angle (naso-facial angle) with the cheek, and ends below in a mobile expanded portion, the ala nasi, which forms the lateral boundary of the naris, and is limited above by a furrow, the alar sulcus. The skin of the nose is thin and movable over the root, but thick and adherent over the apex and alæ, where it contains numerous large sebaceous glands.
The arterial supply of the external nose is derived from the external maxillary and ophthalmic arteries, and its veins open into the anterior facial vein and communicate with the ophthalmic vein. Its principal lymph vessels follow the course of the anterior facial vein and open into the submaxillary lymph glands. From the root of the nose one or two vessels run laterally in the upper eyelid and end in the upper anterior auricular lymph glands, while a third group runs below the orbit to the lower anterior auricular lymph glands. Its muscles are supplied by the facial nerve, and the skin covering it is supplied by the infra-trochlear and naso-ciliary branches of the ophthalmic nerve and the infra-orbital branch of the maxillary nerve.
The external nose presents great variety as to its size and shape, and certain well-defined types, such as aquiline, Grecian, etc., are described. The relation which its breadth, measured across the alæ, bears to its length, measured from root to apex, is termed the nasal index, and is expressed thus :
greatest breadth x 100 greatest length.
In white races this index is below 70 (leptorhines); in yellow races, between 70 and 85 mesorhines); and in black races, above 85 (platyrhines).
Five chief cartilages are concerned in the formation of the nose; they are the lateral and greater alar cartilages, on each side, and the cartilage of the septum.
Cartilago Nasi Lateralis. — The lateral cartilage (Figs. 669, 670) is triangular in shape and is situated immediately below the nasal bone. Its posterior edge is thin and is attached to the maxilla and the nasal bone; its anterior edge is thick, and its superior part is directly continuous with the cartilage of the septum ; its inferior margin is joined, by fibrous tissue, to the upper edge of the greater alar cartilage.
Cartilago Alaris Major.—The greater alar cartilage (Figs. 668, 669, 670) encircles the anterior part of the nostril and assists in keeping it open. It consists
of a lateral and a medial crus, which are continuous with each other in a rounded angle at the apex of the nose. The lateral crus is oval in shape and is attached to the lateral cartilage and the maxilla by fibrous tissue. Above and behind it are two or three lesser alar cartilages, while sometimes a horizontal furrow cuts off a narrow linear part from its superior margin. The inferior edge of the lateral crus does not descend as far as the opening of the nostril, the ala being there devoid of cartilage and composed of fatty and connective tissue covered with skin. The medial crus (Fig. 668) bounds the medial wall of the nostril and lies in the septum mobile, below the anterior part of the cartilage of the septum. The medial crura of the two cartilages are separated, in front, by a notch which corresponds with the apex of the nose, and the posterior end of each curves slightly lateralwards and ends in a rounded extremity.
Cartilago Septi Nasi.—The cartilage of the septum (Fig. 668) is of an irregularly quadrilateral form. Its postero-superior edge is attached to the perpendicular lamina of the ethmoid; its postero-inferior margin to the vomer and the maxillæ. Its antero-superior border is thick, and is fixed above to the back of the internasal
Lesser alar cartilages
-tissue of ala
suture; immediately below the level of the nasal bones it is continued, on each side, into the lateral cartilages, which may be looked upon as its wing-like expansions. The inferior part of this border is separated by à fissure from the lateral cartilage, and extends downwards between the greater alar cartilages, to which it is attached by fibrous tissue; in this fibrous tissue a small accessory cartilage is usually seen on either side of the median plane. Its anteroinferior border is short, and is attached by fibrous tissue to the medial crura of the greater alar cartilages,
Fatty while its anterior angle is rounded and does not reach as far as the apex of the nose.
The lowest part of the nasal septum is not formed by the septal cartilage, but by the medial crura of the greater alar cartilages and by the integument, and, being freely movable, is termed the septum mobile nasi. The cartilage of the septum may be prolonged backwards (especially in children) as a narrow process, the processus
FIG. 669.-PROFILE VIEW OF THE BONY AND CARTI. sphenoidalis, into the angle between
LAGINOUS SKELETON OF THE EXTERNAL NOSE. •the vomer and ethmoid ; this process varies from 4 to 6 mm. in width, and sometimes reaches as far as the body of the sphenoid.
On either side of the inferior edge of the cartilage of the septum, and seen best in a frontal section of the nose, is a narrow band of cartilage, the vomero-nasal cartil
age; it measures from 6 to 12 mm. Nasal bone in length, and is attached to the Frontal process of maxilla
The nasal cavity (Fig. 672) is divided by the nasal septum into a right and a left nasal cavity, which extend from the nostrils in front to the choanæ behind, and open, through the choanæ, into the nasal part of the pharnyx. Their bony boundaries are described in the section on Osteology (p. 183). On the lateral wall of each are found the orifices of the frontal, ethmoidal, sphenoidal, and maxillary sinuses, together with that of the naso-lacrimal duct.
Immediately above the aperture of the nostril is a slightly
FIG. 670.-FRONT VIEW OF THE BONY AND CARTILAGINOUS
SKELETON OF THE EXTERNAL NOSE.