Part 59

The liver is divided by the dorsoventral suspensory ligament into the =right= and =left= lobes, and each half is again divided into lobes. On the left is a small =left median= (b) and a larger =left lateral= lobe (a). The left lateral (a) extends caudad with a thin edge which covers the greater part of the ventral surface of the stomach. On the right there is a large =right median= (or =cystic=) lobe (c, c′). Its cranial surface is dome-shaped and fitted against the right two-thirds of the caudal surface of the diaphragm. Its ventral edge is thin, its dorsal edge thick, and its caudal surface marked by a deep dorsoventral cleft in which lies the gall-bladder (Fig. 101, f). Dorsad and caudad of the cystic lobe is the =right lateral= lobe (d, d′), which is deeply cleft. Its elongated caudal division (d′) extends in a point to the caudal end of the right kidney and is adapted to the medial half of its ventral surface. Its smaller and more compact cranial division (d) ends ventrally in a thin edge. It lies between the caudal division (d′) and the cystic lobe (c), and its dorsal surface is adapted to the suprarenal body. The =caudate= or =Spigelian= lobe (Fig. 101, e) is an elongated, triangular, pyramidal lobe. It lies in the omental sac and partly closes the foramen epiploicum (foramen of Winslow). At its base it is connected with the caudal division of the right lateral lobe (d′).

a, left lateral lobe; b, left median lobe; c, right median lobe; d, d′, right lateral lobe; e, gall-bladder; f, opening of posterior vena cava, with the smaller openings of the hepatic veins.]

a, left lateral lobe; b, left median lobe; c, c′, right median (or cystic) lobe; d, d′, cranial and caudal divisions of the right lateral lobe; e, caudate lobe; f, gall-bladder; g, cystic duct; h, hepatic ducts; i, common bile-duct; j, portal vein; k, part of duodenum.]

The =Gall-bladder= (Fig. 101, f) is pear-shaped and lies in a cleft on the caudal (or dorsal) surface of the right median lobe (c, c′) of the liver. Its larger end is directed caudad (or ventrad) and is free. By one surface it is in contact with the liver and not covered by peritoneum, while the other surface is covered by peritoneum. The peritoneum in passing from the larger free end to the liver forms one or two ligament-like folds. By its smaller end the gall-bladder is continuous with the =cystic duct= (g). This duct is about three centimeters long and has a sinuous course. At its distal end it is joined by two (or more) =hepatic ducts= (h), bringing the bile from the lobes of the liver. The relation of these to the cystic duct varies. They may open into it by a common trunk or separately. Of these hepatic ducts one is made up by the junction of smaller hepatic ducts from the left half of the liver and the left half of the cystic lobe, while the other is similarly formed by smaller ducts from the right half of the cystic lobe, from both divisions of the right lateral lobe and from the caudate lobe. The duct formed by the junction of the hepatic and cystic ducts is the =common bile-duct= (=ductus communis choledochus=) (i). It passes in the free right border of the gastroduodenal omentum to the duodenum (k) and opens into it by way of the ampulla of Vater, in common with the pancreatic duct, at a point on the dorsal surface of the duodenum and about three centimeters from the pylorus.

=Pancreas.=--The pancreas (Fig. 102, a) is a flattened, closely lobulated gland of irregular outline, about twelve centimeters long, varying in width from one to two centimeters. It is bent nearly at right angles at about its middle. One of the halves (a′) into which it is divided by its bend lies in the descending limb of the great omentum, and is near the greater curvature of the stomach (d) and parallel to it. The free end of this half is in contact with the spleen (e). The other half (a) lies in the duodenal omentum between the limits of the duodenal U (c) and reaches to the bottom of the U. The pancreas has two ducts. The larger =pancreatic duct= (b) (sometimes known as the duct of Wirsung) collects the pancreatic fluid from both halves of the gland, the ductlets from each half uniting to make two larger ducts, which then unite near the angle of the gland to make the pancreatic duct. This is short and broad and opens into the ampulla of Vater together with the common bile-duct. The =accessory pancreatic duct= (duct of Santorini) opens into the duodenum about two centimeters caudoventrad of the ampulla of Vater. It is formed by the union of branches which anastomose with those of the pancreatic duct. It is apparently sometimes lacking.

The œsophagus has been cut and the stomach turned caudad, so that the dorsal surface of the stomach and the ventral surface of the duodenum are seen. a, pancreas (a, duodenal portion; a′, gastric portion); b, pancreatic duct; c, duodenum; d, stomach; e, spleen.]

=Spleen. Lien.=--The spleen (Fig. 102, e) is a deep red, flattened, elongated gland belonging to the lymphatic system. One of its ends, the left, lies against the free end of the gastric half of the pancreas and is broader than the other end. The spleen is curved and is suspended in the descending limb of the great omentum so that it follows the greater curvature of the stomach (d).

III. RESPIRATORY ORGANS. APPARATUS RESPIRATORIUS.

The =organs= of =respiration= consist of the =nasal cavity=, the =nasopharynx=, the =pharynx= (also a food-passage), the =larynx= (also the organ of the voice), the =trachea=, the =bronchi=, and the =lungs=. With them are usually described also the =thyroid= and =thymus glands=.

1. =The Nasal Cavity. Cavum nasi.=--The osseous framework of the nasal cavity has already been described (page 59), and in connection with this description the boundaries of the cavity and its connections with other cavities have been given. It consists essentially of a large cavity bounded by the facial bones and divided by a longitudinal partition into two lateral halves. The two cavities thus formed are nearly filled by (1) the labyrinths of the ethmoid (ethmoturbinals), (2) the superior nasal conchæ or nasoturbinals, projecting into the dorsal part from the ventral surface of the nasal bones, and (3) the inferior nasal conchæ, or maxilloturbinals, projecting into the ventral portion from the medial surfaces of the maxillaries.

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.

a, cartilage of the median septum; b, “wings”; c, ridge formed by internal portion of wing.]

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).

b, ceratohyal; c, epihyal; d, stylohyal; e, tympanohyal; f, thyrohyal. 1, thyroid cartilage; 2, epiglottis; 3, cricoid cartilage; 4, arytenoid cartilage; 5, cricothyroid ligament; 6, thyrohyoid ligament; 7, trachea.]

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.