Part 23

There remain to be considered, in addition to the bones, the =cartilaginous= framework of certain parts of the nose, and the =mucous membrane=. The lamina perpendicularis is continued by cartilage, especially craniad, in such a way as to make a complete septum separating the two cavities. This septum extends from the septum of the external nose caudad to the lamina cribrosa, and from the internasal suture ventrad to the vomer and the suture of the premaxillaries. All parts of the nasal cavity are lined by mucous membrane. This is continuous at the nares with the integument, while at the choanæ it passes into the mucosa of the pharynx. It covers the conchæ nasales and the labyrinths of the ethmoid, passing into the cellules of the latter. Owing to the crowding together of the conchæ nasales and the labyrinths the nose is almost completely filled, only three narrow passageways being distinguishable. The ventral one of these, known as the =ventral= or =inferior meatus= of the nose, lies ventrad of the inferior nasal concha, next to the nasal septum. It passes caudad beneath the horizontal plate formed by the vomer and ethmoid, and opens caudad at the =choanæ= into the nasopharynx. The =dorsal= or =superior meatus= of the nose lies just ventrad of the superior nasal concha, next to the median septum; it leads to the frontal sinus, the lamina cribrosa, and the caudal parts of the ethmoid. The =middle meatus= of the nose has almost disappeared in the cat; it is simply the narrow space between the superior and inferior meati. It is practically filled by the ethmoid, into the cells of which it leads. The mucosa is continued from the nasal cavity into the sinuses of the frontals and presphenoid.

At the sides of the nasal septum, near the ventral edge, and about one to one and a half centimeters caudad of the nares, there is on each side a small curved cartilaginous tube, about one centimeter or less in length. This, the =vomeronasal= organ, or =organ= of =Jacobson=, lies against the septum, between it and the mucosa. It begins at the incisive canal or anterior palatine foramen, in the roof of the mouth, curves thence caudodorsad close against the side of the nasal septum, and ends blindly in the nasal cavity.

The =nares= or cranial openings of the nasal cavity are supported by a number of cartilages which form the framework of the =snout= or external nose. The cartilaginous continuation of the lamina perpendicularis extends some distance craniad of the tips of the nasal and premaxillary bones, forming the septum of the external nose (Fig. 103, _a_). From the dorsal edge of this projecting cartilaginous septum, two thin cartilaginous wings (_b_) extend laterad, forming the dorsal wall of the narial opening. Each then turns ventrad to form the lateral wall of the opening, but does not form the ventral floor; instead it curves mediad and finally dorsad, thus extending from the floor of the opening as a prominent ridge (_c_) within the nares. The cartilaginous wing is thus rolled into a sort of spiral, ending with a free edge within the narial opening. From the ventral edge of the median cartilaginous septum there are likewise lateral extensions, which form part of the ventral boundary of the nares; these do not quite reach the ventral parts of the dorsal wings, however, so that a small part of the narial opening is not bounded by cartilage. A section of the narial cartilages is shown in Fig. 103. The wings from the dorsal edge of the septum do not extend quite to the cranial tip of the nose, so that a notch is formed on the ventrolateral side of the nares; a section in this region would therefore differ from that figured.

The framework of the external nose thus formed is covered externally by thick hairless skin, containing many glands. From the ventral end of the internarial septum a groove passes ventrad, partly dividing the upper lip.

The inner surfaces of the cartilages are covered by the =mucosa=, which forms a number of ridges. The narial opening is almost completely divided by the prominent ridge which is supported by the free edge (_c_) of the cartilaginous wing above described. This free edge is covered by a thick layer of mucosa, and the entire ridge so formed is continuous caudad with the inferior nasal concha. On the medial side of the partial partition formed by this ridge and near the ventral side there begins a slight distance caudad of the outer opening another ridge, supported by the ventrally incurved portion of the cartilaginous ring. This soon becomes a thick swelling; beneath it opens the lachrymal canal, and that part of the passageway that lies ventromediad of it is the beginning of the inferior meatus of the nose. About one or two centimeters caudad of the external opening a third ridge projects from the dorsolateral wall of the cavity toward the large ridge first described; dorsomediad of it is a narrow passage which is the entrance to the =superior meatus= of the nose.

The =olfactory mucosa=, or that part to which the olfactory nerve is distributed, and which therefore acts as the sensory surface, is confined to the dorsocaudal parts of the nasal cavity, in the region occupied by the cells of the ethmoid. The air penetrates to this region probably only by a definite act of snuffing, the inferior meatus serving as the usual passageway of air to the lungs.

The =nasopharynx= and =pharynx= are considered under the alimentary canal (page 231).

2. =The Larynx.=

The larynx is the enlarged upper end of the air-passage which leads from the pharynx to the lungs. It is a box composed of pieces of cartilage connected by ligaments and moved by muscles, and it is lined by mucous membrane. At the root of the tongue about one centimeter caudad of the body of the hyoid bone is the triangular leaf-like =epiglottis= (Fig. 95, _e_, page 227). When food is taken this closes the opening into the larynx, and the food passes over it into the œsophagus. It is so curved that its apex is directed craniad. A fold of mucous membrane, the =plica glossoepiglottica= or =frenulum= of the epiglottis, extends from the middle of its cranial surface to the root of the tongue, and on each side of this fold is a depression.

From each side of the base of the epiglottis a fold, =plica aryepiglottica= (Fig. 95, _f_), extends caudad to the base of the arytenoid cartilage (_g_). Dorsad of this fold and separated from it by a depression is a ridge which marks the position of the caudal hyoid cornu. The plicæ aryepiglotticæ (_f_) and the epiglottis (_e_) form the boundaries of the =aditus laryngis=, or opening into the larynx.

The cavity of the larynx is divided into three portions. The upper one of these is the =vestibule= of the larynx. It is bounded caudad by two folds of mucosa (_i_) that stretch from the caudal surface of the epiglottis near its base to the tips of the arytenoid cartilages. These folds are the =false vocal cords= (_i_). Their vibration is said to produce purring. Caudad of the false vocal cords two folds of the mucosa stretch from the apices of the arytenoid cartilages to the thyroid cartilages, near the base of the epiglottis. These folds are nearer the median plane than the false vocal cords. They are the =true vocal cords= (_j_). The =middle portion= of the laryngeal cavity is that between the true and the false vocal cords. It is produced laterally into a very small pouch or pocket on each side, the =ventriculus=. The narrow slit between the true vocal cords is the =glottis= (_h_). It can be narrowed and widened by the action of muscles. The vocal cords, which bound it, are set vibrating by currents of air transmitted from the lungs, and the voice-sounds are thus produced. The =caudal portion= (inferior portion) of the laryngeal cavity is that between the glottis and the first tracheal cartilage. It is narrowed near the glottis.

CARTILAGES OF THE LARYNX (Fig. 104).--There are three unpaired cartilages, the =thyroid= (1), =cricoid= (3), and =epiglottic= (2), and two paired cartilages, the =arytenoids= (4).

The =thyroid= cartilage (=cartilago thyreoidea=) (1) has nearly the form of a visor of a cap, but is relatively broader at its ends than a cap visor. It forms about two-thirds the circumference of a circle, and is so situated that it embraces the other cartilages ventrally and laterally. Its caudal and cranial borders are oblique to its caudocranial axis and are directed dorsocaudad. To the middle of the cranial border is attached the epiglottic cartilage (2), and the whole cranial margin is connected by membrane (6) to the body and caudal cornua (_f_) of the hyoid bone. The dorsal border projects craniad into a considerable cornu which is attached to the free end of the caudal hyoid cornu (_f_). The border also projects caudad into a process which articulates with a facet on the lateral surface of the cricoid cartilage (3). In the middle of the dorsal surface is a longitudinal ridge for attachment of the vocal cords and origin of the thyroarytenoid muscles (Fig. 105, _d_), and at the caudal end of this ridge the caudal border presents a considerable rounded notch.

The =cricoid cartilage= (=cartilago cricoidea=) (3) has the form of a seal ring with its broader part dorsad. The broad dorsal part of the ring is partly embraced by the wings of the thyroid cartilage (1). Its caudal border is undulating and nearly at right angles to its caudocranial axis. It is connected by membrane with the first tracheal ring. The cranial border is oblique to the long axis and lies in a plane which passes from the ventral side craniodorsad. Near the median line on each side it articulates by an oblique facet with one of the two arytenoid cartilages (4). Near the midventral line the cricothyroid ligament (5) is attached and it stretches thence to the midcaudal notch of the thyroid cartilage (1).

The outer surface presents at the middle of each side a facet for articulation with the thyroid, and is marked in the median dorsal line by a ridge for the posterior cricoarytenoid muscle (Fig. 105, _b_).

The =arytenoid cartilages= (=cartilago arytenoidea=) (Fig. 104, 4) are triangular pyramids with base and sides nearly equilateral triangles. One side articulates with an oblique facet on the cranial border of the cricoid near the mid-dorsal line. Near the opposite apex is attached the vocal cord. At the lateral angle of the base is attached on its dorsal side the posterior cricoarytenoid muscle (Fig. 105, _b_), and on its ventral side the thyroarytenoid (Fig. 105, _d_) and lateral cricoarytenoid (Fig. 105, _c_) muscles. The movements produced by these muscles carry the arytenoid ends of the vocal cords toward or from the median plane and thus open or close the glottis.

The =epiglottic cartilage= (Fig. 104, 2) is flexible (fibro-cartilage) and of a cordate form. It is so curved that its caudal surface is convex dorsoventrally and concave from side to side. It supports the epiglottis. Its cranial surface presents a slight median ridge for attachment of muscles. By its base it is attached to the midventral part of the cranial border of the thyroid cartilage (1). Its position varies so that it either stands erect with its apex directed craniad to allow the passage of air to the lungs, or, as in the act of swallowing, it is turned caudad over the aditus laryngis so as to allow food to pass over it and into the œsophagus.

The =vocal cords= are two fibrous elastic bands. Each is attached at one end to the apex of the arytenoid cartilage, and at the other end to the median ridge on the dorsal surface of the thyroid. Each supports a projecting fold of mucous membrane, the vibration of which causes the voice.

MUSCLES OF THE LARYNX.--1. Muscles moving the entire larynx.

A. =Elevators. M. thyreohyoideus= (Fig. 96, _p_, page 229).--A flat band on the lateral side of the larynx.

_Origin_ on the lateral part of the caudal border of the thyroid cartilage.

_Insertion_ on the medial two-thirds of the caudal border of the caudal cornu of the hyoid.

_Action._--Raises the larynx.

The stylohyoid (Fig. 65, _d_, page 109) and the median and inferior constrictors (Fig. 96, _j_ and _k_) of the pharynx, already described, have the same action.

B. =Depressors.=--The sternothyroid (Fig. 65, _g′_), already described (p. 141).

2. Muscles which move the parts of the larynx one upon another.

A. =Muscles on the Outer Surface of the Larynx.=

=M. cricothyreoideus= (Fig. 96, _n_).--A broad flat band which with its fellow covers the ventral surface of the cricoid cartilage and the cricothyroid ligament.

_Origin._--The lateral half of the ventral surface of the cricoid cartilage. The muscles diverge so as to leave a part of the cricothyroid ligament between them.

_Insertion._--The ventral part of the caudal border of the thyroid cartilage laterad of the median ventral notch.

=M. cricoarytenoideus posterior= (Fig. 105, _b_).--The two muscles cover the dorsal surface of the larynx.

_Origin._--From the dorsal part of the caudal border of the cricoid cartilage (3) and from its median dorsal crest. The fibres pass craniolaterad, converging. The lateral fibres are nearly longitudinal in direction.

_Insertion._--The dorsal border of the caudal end of the arytenoid cartilage (4).

_Action._--Moves the arytenoid on its oblique articulation with the thyroid. The apex of the arytenoid is thus carried laterad, and the vocal cords are separated so as to widen the glottis.

=M. arytenoideus transversus= (Fig. 105, _a_).--A small unpaired muscle running transversely between the caudal ends of the arytenoid cartilages, just beneath the mucous membrane and parallel to the cranial border of the cricoid cartilage.

=M. glossoepiglotticus.=--A small longitudinal muscle, lying by the side of its fellow in the frenulum of the epiglottis.

_Origin._--The median fibrous septum of the tongue.

_Insertion._--The dorsal (cranial) surface of the epiglottic cartilage in the median line near its attached border.

_Action._--Draws the epiglottis craniad.

=M. hyoepiglotticus= is a small bundle of parallel fibres lying in the frenulum of the epiglottis craniad and dorsad of the body of the hyoid bone.

_Origin._--The lateral end of the cranial surface of the body of the hyoid. The two muscles pass craniodorsad, converging, and unite with the preceding.

_Insertion_ with the preceding. (The lateral portion of the muscle may continue craniad to the tongue as a second part of the hyoglossus.)

_Action._--Like the preceding.

B. =Muscles on the Inner Surface of the Cartilages of the Larynx.=

=M. thyreoarytenoideus= (Fig. 105, _d_).--The thyroarytenoid is a triangular, flat muscle of considerable size. It lies within the wing of the thyroid cartilage (2), and its fibres are nearly dorsoventral in direction.

_Origin._--The median longitudinal crest on the dorsal surface of the thyroid cartilage (2).

_Insertion._--The fibres converge to the insertion into the cranial lip of the laterocaudal angle of the arytenoid cartilage (4).

_Action._--Turns the arytenoid on its oblique articulation so as to close the glottis.

=M. cricoarytenoideus lateralis= (Fig. 105, _c_).--Triangular, a little smaller than the preceding, caudad of which it is situated, so that it also is covered by the wing of the thyroid.

_Origin._--The lateral part of the cranial border of the cricoid cartilage (3). The fibres converge, passing dorsad.

_Insertion._--The caudal lip of the laterocaudal angle of the arytenoid cartilage (4).

_Action._--Similar to the preceding, so that it closes the glottis.

3. =The Trachea= (Fig. 105, 5; Fig. 106, _a_).--The trachea is that part of the air-passage which extends from the larynx to the bronchi (Fig. 106). It is a straight tube composed of a lining mucosa with ciliated epithelium, and a connective-tissue covering which encloses supporting cartilages. Each tracheal cartilage is incomplete dorsally where it lies against the œsophagus, the gap between the two free ends of each ring being filled with muscular and connective tissue. As a result of this the diameter of the trachea is not fixed, but can be increased and diminished. The first ring is broader than the others. Where the œsophagus leaves the median line, the dorsal surface of the trachea lies against the longus colli muscles (Fig. 72, _g″_). Its ventral surface is against the sternohyoid (Fig. 65, _e_) and sternothyroid (Fig. 65, _g′_) muscles. Its lateral surfaces are partly covered by the thyroid gland (Fig. 96, 6), and are in close relation with the carotid artery (Fig. 119, _a_, page 284), the vagus and sympathetic nerves (Fig. 156, _i_), and the internal jugular vein (Fig. 119, _b_). In the thoracic cavity the great vessels coming from the heart lie against the ventral surface of the trachea (Fig. 129, 2). At about the level of the sixth rib the trachea divides into the two main =bronchi= (Fig. 106). Each bronchus is supported by incomplete rings of cartilage like those of the trachea and has otherwise in general the structure of the trachea. In the lungs the bronchi become divided into many branches (Fig. 106), in the manner described in the account of the lungs.

4. =The Lungs. Pulmones.=--Immediately after division of the trachea the two bronchi enter the lungs (Fig. 106). These are two large, much-lobed organs, which fill the greater part of the thoracic cavity. The main lobes of the lungs are completely separated from each other, except in so far as they are connected by the bronchi and connective tissue; the main lobes may also be partly subdivided into secondary lobes that are not thus completely separated. The two lungs are completely separated from each other, except at the =radix=, where they are united by the bronchi; they lie in the right and left halves of the thoracic cavity, with the mediastinal septum between them. The bronchi on entering the lungs divide in the following manner. Each divides at first into two main branches. The cranial branch on the right side is known as the =eparterial= bronchus (_b_), because it lies craniad of the pulmonary artery. All the others are =hyparterial=; i.e., they lie caudad of the pulmonary artery. The right cranial bronchus does not further subdivide into large bronchi, but gives off numerous small branches. The right caudal bronchus divides into three main branches. There are thus four main branches of the right bronchus. The left cranial bronchus divides into two main branches; the left caudal bronchus continues caudad as a main trunk giving off small branchlets. Of the left bronchus there are thus but three main subdivisions. The main subdivisions of the bronchi on the two sides correspond, as will be seen, with the lobulation of the lungs.

The =right= lung (Fig. 106, 1-4) is slightly larger than the left (1′-3′). It divides into three smaller proximal lobes (1-3), and one large distal one (4). The most cranial one of the proximal lobes (1) is sometimes partly subdivided. The third one of the proximal lobes (3) lies mediad of the others; it is partly subdivided and one-half projects into a pocket in the mediastinum, so that it comes to lie across the middle line, extending a short distance onto the left side. This lobe is frequently called the =mediastinal= lobe (3). The caudal lobe (4) of the right lung is large and flat, containing about half the substance of the lung.

The =left= lung is divided into three main lobes (1′-3′); the two cranial ones (1′-2′) are, however, partly united at the base, so that they may be considered subdivisions of but a single lobe; thus the left lung has but two distinctly separated lobes.

Each lung is attached to the aorta, vertebral column, and diaphragm by a fold of pleura, the =pulmonary ligament=. This is broadest at the caudal lobe of each lung. Each pulmonary ligament is double, being formed of two sheets of the pleura.

=The Thyroid Gland. Glandula thyreoidea= (Fig. 96, 6).--The thyroid gland consists of two lateral lobes (6) and a median lobe or =isthmus= (7). Each lateral lobe (6) is an elongated, flattened, lobulated mass with round ends. It is about two centimeters long and about one-fourth as broad. It lies at the side of the trachea (4), dorsad of the lateral margin of the sternohyoid muscle. Its cranial end is at the level of the caudal border of the cricoid cartilage. The =isthmus= (7) is a delicate band two millimeters wide which connects the caudal ends of the two lateral lobes. It passes ventrad of the trachea and in close contact with it. The thyroid has no duct.

=The Thymus Gland. Glandula thymus= (Fig. 107, _d_).--The thymus gland is best developed in young kittens; in the adult cat it has partly or almost completely degenerated. It is an elongated, flattened organ, of a pinkish-gray color, which lies in the mediastinal cavity, between the two lungs and against the sternum. It extends caudad as far as the heart (_a_), overlying the pericardium at its posterior end. At its cranial end it projects, when well developed, a short distance (about one centimeter) outside of the thoracic cavity into the neck region. The caudal end is forked, and the left lobe thus formed is usually larger than the right. The cranial end may also show indication of a division into two lobes, but this is frequently not the case.

IV. THE UROGENITAL SYSTEM. APPARATUS UROGENITALIS.

1. The Excretory Organs.

KIDNEY. REN (Figs. 108 and 109).