The alimentary canal may be divided into mouth, pharynx, œsophagus,
stomach, small intestine, and large intestine. With these are associated
certain accessory structures,--the salivary glands, the liver, and the
pancreas. The spleen, though not belonging to the digestive system, is
usually described in connection with it. The respiratory organs are
almost throughout in close relation with the organs of the digestive
system.
1. =The Mouth. Cavum oris.=--The mouth cavity extends from the lips to
the pharynx. It is narrower toward the lips, broadens caudad as far back
as the last teeth, then becomes narrowed to form the =isthmus faucium=,
by which it communicates with the pharynx. The mouth cavity is divisible
into the =vestibule= of the mouth (=vestibulum oris=), which comprises
that part outside the jaws proper, bounded externally by the lips and
cheeks, and the =mouth cavity proper= (=cavum oris proprium=), which
lies within the teeth. That portion of the vestibule which is bounded by
the cheeks is sometimes farther distinguished as the =buccal cavity=.
The entire mouth cavity (except the teeth) is lined by the mucous
membrane or =mucosa=.
The =lips= (=labia oris=) are thick folds of skin bounding the entrance
to the mouth cavity. The outer surface is covered with hair; the inner
surface is covered with the mucous membrane. The upper lip is marked in
the median line by a deep external groove which extends upward to the
septum of the nose. Along the inner surface of this groove the lip is
closely united to the jaw by a thick fold, the =frenulum= of the upper
lip. For some distance on each side of the frenulum the inner surface of
the lip bears numerous large papillæ. The =lower= lip is also united to
the jaw by a frenulum in the median line; it is again united to the jaw
just caudad of the canine tooth, in the space between the latter and the
first premolar. Caudad the two lips pass into each other (forming the
=commissura labiorum=), and unite with the cheek. The muscles of the
lips have been described (page 105).
The =cheeks= (=buccæ=) in the cat are comparatively thin and small,
extending from the lips caudad to the ramus of the mandible. The outer
surface is covered with hair; the inner surface is smooth and somewhat
folded. The buccal cavity is rather small. On the inner surface of the
cheek open the duct of the parotid (Steno’s duct), the ducts of the
molar gland, and that of the infraorbital gland.
The roof of the mouth cavity is formed by the hard and soft palates. The
hard palate (=palatum durum=) forms the cranial part of the roof; it is
supported by the palatal plates of the maxillary and palatine bones.
The mucosa of the hard palate is elevated to form seven or eight curved
transverse ridges, which are concave caudad. Between the ridges are rows
of papillæ. In front of the most cranial ridge is a papilla in the
middle line, and at each side of the papilla is the opening of a small
duct (the =incisive duct= or =Stenson’s duct=), which leads dorsad
through the incisive foramen to the =vomeronasal= organ (or =organ of
Jacobson=), which lies on the floor of the nasal cavity. Caudad of the
hard palate the roof of the mouth is formed by the soft palate or =velum
palatinum=, described below.
The floor of the mouth cavity is formed chiefly by the tongue, which
extends as far caudad as the isthmus faucium. Ventrad of the free edge
of the tongue the mucosa forms a prominent median vertical fold which
unites the tongue with the floor of the mouth beneath it; this fold is
the frenulum linguæ. On each side of the median line at the cranial
border of the floor of the mouth is a prominent papilla, at the apex of
which open the ducts of the submaxillary and sublingual glands, the
former on the lateral side of the apex, the latter on the medial side.
The sides of the mouth cavity are formed by the teeth and the gums,
covering the alveolar borders of the mandible, maxillaries, and
premaxillaries.
The mouth cavity presents further for examination the =glands=, the
=teeth=, the =tongue=, and the =soft palate=.
THE GLANDS OF THE MOUTH (GLANDULÆ ORIS).--There are five pairs of
salivary glands which open into the mouth cavity.
1. The =parotid gland= (=glandula parotis=) (Fig. 65, 1; Fig. 131, 10)
is flattened, rather finely lobulated, and lies ventrad of the external
auditory meatus and beneath the dermal muscles. Its cranial border
follows the caudal border of the masseter muscle and overlies it
somewhat; its caudal border is about three centimeters caudad of the
border of the masseter. Its borders are unevenly lobed. The parotid duct
(=ductus parotideus=: frequently called =Stenon’s= or =Steno’s duct=) is
formed by the union of several smaller ducts near the ventral end of the
cranial border of the gland. It passes craniad imbedded in the fascia
covering the masseter. At the cranial border of the masseter it turns
inward and lies close against the mucous membrane of the mouth, so that
from the inside of the mouth it appears as a white ridge on the mucosa.
It opens on the inside of the cheek opposite the most prominent cusp of
the last premolar tooth. Along the course of the parotid duct in some
cases one or more small =accessory parotid glands= are found.
2. The =submaxillary gland= (=glandula submaxillaris=) (Fig. 65, 2, page
109, and Fig. 131, 11) is approximately kidney-shaped. Its surface is
nearly smooth, the lobulations not being apparent externally. It lies
ventrad of the parotid, at the caudal edge of the masseter muscle, just
caudad of the angular process of the mandible. The posterior facial vein
(Fig. 131, =b=) crosses its outer surface, and its cranioventral border
is hidden by two lymphatic glands (Fig. 131, 12) lying at the sides of
the anterior facial vein. The =submaxillary duct= (=ductus
submaxillaris=, frequently called =Wharton’s duct=) leaves the inner
surface of the gland and passes beneath the digastric and mylohyoid
muscles and against the outer surface of the styloglossus. From the
point where the styloglossus passes into the tongue the duct continues
craniad close against the oral mucosa and parallel to the mandibula. It
is accompanied by the duct of the sublingual, which lies at first dorsad
of it and then mediad. It opens at the apex of the prominent papilla
which lies at the side of the middle line at the cranial end of the
floor of the mouth.
3. The =sublingual gland= is elongated and conical in form, with its
base against the submaxillary, of which it appears to be a continuation.
It stretches along the submaxillary duct for about one and one-half
centimeters, lying between the masseter and digastric muscles. The
=sublingual= duct leaves its ventral side, passes close to the
submaxillary duct, at first dorsad and then mediad of it, and opens on
the medial side of the apex of the same papilla with the submaxillary
duct.
4. =The molar gland= (=glandula molaris=) (Fig. 65, 9) lies between the
orbicularis oris and the mucosa of the lower lip. It stretches from the
cranial border of the masseter to a point between the first premolar and
the canine. It is flat, broad caudad, and ends in a point craniad. It
has several ducts which pass straight through the cheek and open on the
mucous surface of the mouth.
5. The =infraorbital= (or =orbital=) =gland= lies in the lateral part of
the orbit on its ventral floor. It is ovoid and about one and one-half
centimeters long and one-third as thick. Its ventral end rests against
the mucosa of the mouth just caudad of the molar tooth. Its duct leaves
the ventral end and opens into the mouth at a point about three
millimeters caudad of the molar tooth.
THE TEETH. DENTES.--The adult cat has thirty teeth, fourteen in the
lower jaw and sixteen in the upper jaw. There are twelve =incisors=,
four =canines=, ten =premolars=, and four =molars=. The tooth formula
for the cat is then
3 - 3 1 - 1 3 - 3 1 - 1
_i_-----, _c_-----, _pm_-----, _m_-----.
3 - 3 1 - 1 2 - 2 1 - 1
The teeth are implanted in the alveolar borders of the premaxillaries,
maxillaries, and mandible. In each tooth can be distinguished the
=root=, imbedded in the socket of the bone, the =crown=, which projects
above the gums, and a narrow =neck= connecting the two. The root is
composed of one or more separate fangs; the crown bears one or more
points or cusps.
At the cranial end of each jaw are six =incisor= teeth (Figs. 93 and 94,
_a_). These are imbedded in the alveolar borders of the premaxillaries
and the mandible. The incisor teeth are small, with a crown bearing a
sharp edge which is notched so as to form three minute cusps. The root
of each has a single fang. The lateral incisors are the largest in each
jaw, and those of the upper jaw are larger than those of the lower.
[Illustration: FIG. 93.--UPPER JAW, WITH ROOTS OF THE TEETH LAID BARE.
_a_, incisors; _b_, canine; _c_, first premolar; _d_, second premolar;
_e_, third premolar; _f_, molar.]
Caudad of the incisors, and in the upper jaw separated from them by a
slight interval, are the =canines= (_b_), two in each jaw. These are
long, strong, pointed teeth, deeply imbedded in the mandible and
maxillaries, their large sockets causing a rounded swelling on the
external surface of the bones. Each has a single fang and a single cusp.
When the mouth is closed the upper canines lie laterocaudad of the lower
ones.
Caudad of the canines there is in each jaw a considerable interval free
from teeth: this is called the =diastema=. Caudad of the diastema are
the =premolar= teeth, three pairs (_c_, _d_, _e_) in the upper jaw and
two pairs (_c_, _d_) in the lower jaw. These teeth are compressed
sideways, and those of the lower jaw fit inside of those of the upper
jaw. In the =upper= jaw (Fig. 93) the first premolar (_c_) is small and
usually has but a single cusp and a single fang, though occasionally
there is a small supplementary cusp and fang. The second premolar is
larger (_d_); it has a large central cusp, with a single smaller cranial
cusp and two small caudal cusps, making four in all. This tooth has two
fangs. The third premolar (_e_) is the largest tooth in the jaws; it has
three large cusps in longitudinal series and a small cusp lying on the
medial side of the first one in the row. Its root has three fangs. The
=molar= tooth (_f_) of the upper jaw is small and lies caudomediad of
the last premolar. It has two small cusps and two fangs.
[Illustration: FIG. 94.--MANDIBLE, WITH ROOTS OF THE TEETH LAID BARE.
_a_, incisors; _b_, canine; _c_, first premolar; _d_, second premolar;
_f_, molar.]
In the =lower= jaw (Fig. 94) the two premolars (_c_ and _d_) are
similar, the caudal one being a little larger. Each has four cusps; a
single large one, a small one craniad of this, and two small ones caudad
of it. Each has two fangs. The single molar (_f_) is the largest tooth
of the lower jaw; it has two large cusps and two fangs.
=The Deciduous Teeth.=--At birth the cat has no teeth. There appears
later a set of twenty-six teeth: twelve incisors, four canines, and ten
molars (six in the upper jaw and four in the lower). These teeth are
later replaced by the permanent ones above described. The deciduous
teeth of the cat are fully described by Jayne (“Mammalian Anatomy,” vol.
1. p. 319), where also an account is given of the order of appearance of
the teeth.
THE TONGUE. LINGUA (Fig. 95).--The =tongue= is a muscular organ covered
with mucous membrane; in life it is very mobile. It is an elongated
organ, flat above, broadest in the middle, and very slightly narrowed at
each end. It extends from the incisor teeth to the isthmus faucium and
nearly fills the mouth cavity. The caudal third of the tongue forms the
floor of the mouth cavity, so that the tongue has here no ventral
surface, but is directly continuous with underlying organs. It is in
this region that the extrinsic muscles of the tongue (except the
genioglossus) enter it. The cranial two-thirds of the tongue is partly
or entirely free from the floor of the mouth, the cranial one-third
being completely free and movable. In about the middle third the ventral
surface of the tongue is held to the floor of the mouth by the fold
known as the =frenulum linguæ=. The frenulum contains parts of the two
genioglossus muscles, which enter the tongue through it. The ventral
surface and lateral borders of the tongue are smooth, soft, and free
from papillæ. The dorsal surface is raised into papillæ of various
kinds, and has a slight median longitudinal furrow. The caudal part of
the dorsal surface is softer, redder, and marked with papillæ of a
different kind from those of the rest of the tongue. From the caudal
end a small median vertical fold, the =frenulum= (or =plica=)
=glossoepiglottica= passes from the dorsal surface of the tongue to the
cranial surface of the epiglottis.
[Illustration: FIG. 95.--TONGUE, EPIGLOTTIS, AND OPENING OF LARYNX.
_a_, filiform papillæ; _b_, fungiform papillæ; _b′_, very large
papillæ at the sides of the tongue; _c_, circumvallate papillæ; _d_,
tonsils; _e_, epiglottis; _f_, plica aryepiglottica; _g_, arytenoid
cartilages (covered with mucosa); _h_, glottis; _i_, false vocal
cords; _j_, true vocal cords.]
The =papillæ= of the tongue are of three kinds. 1. The very numerous
=filiform papillæ= (_a_) (=papillæ filiformes=); many of them are horny
and tooth-like, with points turned caudad. These are most numerous at
the middle of the free end of the tongue. 2. The =fungiform papillæ=
(_b_) (=papillæ fungiformes=) are found scattered over the surface of
the middle of the tongue caudad of the large filiform papillæ. They are
enlarged at their free ends. There is a prominent row of very large ones
(_b′_) opposite the circumvallate papillæ at the borders of the tongue.
3. The =circumvallate papillæ= (_c_) (=papillæ vallatæ=) are blunt and
each is surrounded by a trench which is bounded in turn by a raised
wall. They are in two rows of two or three each, which converge near the
base of the tongue so as to form a V with the apex directed caudad.
MUSCLES OF THE TONGUE (Fig. 96).--=M. genioglossus= (_f_) passes from
the symphysis of the lower jaw into the tongue and lies beneath (dorsad
of) the geniohyoid (_g_).
_Origin_ from the medial surface of the mandible near the symphysis and
dorsad of the origin of the geniohyoid.
_Insertion._--The fibres pass dorsad, diverging in a fan-like manner and
forming a flat vertical plate closely applied to the muscle of the
opposite side. This plate extends along the caudal three-fourths of the
tongue, i.e. as far as it is attached. The cranial fibres arch craniad
to the tip of the tongue, the caudal fibres arch caudad to the root of
the tongue. Dorsad the muscle is confounded with the muscle of the
opposite side.
_Action._--Draws the root of the tongue forward and the tip backward.
=M. hyoglossus= (_h_).--From the body of the hyoid bone to the tongue.
_Origin._--(1) From the ventral surface of the body of the hyoid laterad
of the geniohyoid (_g_), and (2) by a second head from the ceratohyal.
_Insertion._--Both heads penetrate into the tongue between the
styloglossus (_e_) and the genioglossus (_f_). The fibres intermingle
with those of the styloglossus (_e_) and thus help to form the lateral
parts of the tongue. They finally end in the integument on the dorsum of
the tongue at the sides.
_Action._--Retracts the tongue and depresses it.
=M. styloglossus= (_e_).--From the stylohyal bone to the tongue.
_Origin_ from the mastoid process of the temporal bone, from the
stylomandibular ligament (2) (which connects the border of the external
auditory meatus with the angular process of the mandible) and from the
proximal cartilaginous portion of the cranial cornu of the hyoid bone.
The fibres pass mediad, diverging between those of the digastric and
hyoglossus (_h_) into the lateral part of the tongue.
_Insertion._--The fibres pass toward the tip of the tongue, where the
mass finally ends in a point, the superficial ones gaining insertion
into the integument at the sides of the tongue.
_Relations._--Outer surface with the digastric (Fig. 65, _b_) and
mylohyoid (Fig. 65, _c_). Inner surface with the pterygoideus internus
(Fig. 96, _d_), the cranial cornu of the hyoid, and the tympanic bulla.
_Action._--Retracts the tongue and raises it.
[Illustration: FIG. 96.--MUSCLES OF TONGUE, HYOID BONE, AND PHARYNX.
_a_, M. tragicus lateralis; _b_, M. jugulohyoideus; _c_, M.
pterygoideus externus; _d_, partially cut surface of M. pterygoideus
internus; _e_, M. styloglossus; _f_, M. genioglossus; _g_, M.
geniohyoideus; _h_, M. hyoglossus; _i_, M. glossopharyngeus; _j_, M.
constrictor pharyngis medius; _k_, M. constrictor pharyngis inferior;
_l_, M. stylopharyngeus; _m_, M. sternohyoideus (cut); _n_, M.
cricothyreoideus; _o_, M. sternothyreoideus (cut); _p_, M.
thyreohyoideus. 1, mandible; 1′, angular process of mandible; 2,
stylomandibular ligament; 3, bulla tympani; 4, trachea; 5, œsophagus;
6, thyroid gland; 7, isthmus of the thyroid gland.]
The =intrinsic muscles= of the tongue (those entirely within it) are
attached to its integument at both their ends. There are three sets of
fibres: a longitudinal, a transverse, and a vertical one. These are seen
most readily in cross-sections.
THE SOFT PALATE. VELUM PALATINUM (Fig. 66, page 112).--The soft palate
is the free curtain-like structure which forms the caudal part of the
roof of the mouth. It is attached to the caudal border of the palatal
plates and the ventral border of the perpendicular plates, of the
palatine, and to the pterygoid processes and hamuli of the sphenoid, and
extends some distance caudad of the hamuli. It thus forms a rather long
and narrow curtain separating the caudal part of the nasal cavity from
the mouth. Caudad it ends in a free arched border (Fig. 66, 4) which is
at about the level of the epiglottis, and may lie against the cranial or
the caudal surface of the latter. The narrowed passage bounded by the
margin of the velum palatinum dorsad and the tongue ventrad is the
=isthmus faucium=. From the sides of the velum a short distance from the
caudal border a fold of mucosa passes ventrad to the side of the tongue;
a short distance caudad of this a similar fold passes to the floor of
the pharynx. These folds form the cranial and caudal =pillars of the
fauces=. Between these folds is a shallow pocket, from the bottom of
which there arises a prominent projection or swelling which is one of
the two =tonsils= (Fig. 95, _d_). Each tonsil is a reddish, lobulated
gland, lymphoid in the adult, nearly a centimeter in length, and about
one-third as long as broad, with its long axis craniocaudad.
The velum palatinum consists of two layers of mucous membrane, oral and
nasal, with intervening muscular and connective tissue. The muscles of
the soft palate in the cat are as follows:
=M. tensor veli palatini= (Fig. 66, _d_, _d′_, page 112).
_Origin_ from the ventral surface of the body of the sphenoid between
the foramen ovale and the groove for the Eustachian tube. The muscle
ends in a flat tendon which passes over the hamular process (3) of the
pterygoid bone.
_Insertion_ by spreading out in the soft palate into an aponeurosis
which joins the aponeurosis of the opposite muscle and lies between the
mucous membrane of the mouth and that of the nose.
_Action._--Stretches the palate.
=M. levator veli palatini= (Fig. 66, _e_, _e′_).--A flat triangular
muscle which lies within the tensor.
_Origin_ from the surface of the body of the sphenoid mediad of the
groove for the Eustachian tube, from the styliform process of the bulla
tympani, and in part from the Eustachian tube. The muscle passes caudad,
and its fibres then diverge into the velum palatinum.
_Insertion_ into the velum palatinum, some of the fibres meeting in the
middle line.
_Action_ indicated by the name.
A number of other muscles have been described in the soft palate of the
cat; they are, however, poorly developed and not easily distinguished.
For an account of these, see Stowell, Proceedings of the Am. Soc. of
Microscopists, 1889.
2. =The Pharynx.=--At the caudal end of the mouth cavity the passage
for the food and that for the air cross; at the cranial end the
food-passage (mouth) is ventral, the respiratory passage (nasal cavity)
dorsal. Farther caudad the food-passage (œsophagus) is dorsal, while the
respiratory passage (larynx and trachea) is ventral. In the region of
crossing there is therefore for a certain distance a common passageway
for food and air, and this is known as the =pharynx=. It extends from
the isthmus faucium, at the free caudal margin of the soft palate, to
the beginning of the œsophagus, at the dorsal or caudal margin of the
opening of the larynx. The dorsal wall of the pharynx is separated from
the base of the skull and the centra of the cervical vertebræ only by
intervening muscles (=longus capitis=, =levator scapulæ ventralis=, and
=longus colli=, Fig. 72, page 143). Its lateral and ventral walls are
supported by the hyoid bone and the cartilages of the larynx.
Craniad the pharynx continues, usually, without break into the cavity
lying dorsad of the soft palate. But at the time of swallowing the free
edge of the soft palate is pushed dorsad against the dorsal wall of the
pharynx, while the caudal part of the pharynx is drawn craniad, so as to
form a cavity continuous with that of the mouth. In this way the cavity
above the soft palate is completely separated at the time of swallowing
from the rest of the pharynx. This separated portion is known as the
=nasopharynx=: it is strictly a portion of the respiratory passage, as
the food does not pass into it. The nasopharynx is continuous craniad by
the =choanæ= with the nasal cavity; it forms a horizontal tube between
and ventrad of the perpendicular plates of the palatine bones, and has
the same craniocaudal extent as the soft palate. Its dorsal wall lies
against the basis cranii and the longus capitis muscles; its lateral
walls against the pterygoid muscles and the perpendicular plates of the
palatine bones; its ventral wall is the soft palate. At the middle of
its length, at the junction of its dorsal and lateral wall, are two
longitudinal slits about three millimeters long. These are the medial
openings of the =Eustachian tubes=, by which the nasopharynx
communicates with the tympanic cavity.
The =pharynx proper=, situated caudad of the nasopharynx, is smaller
than the latter. It is bounded craniad by the epiglottis and the margin
of the soft palate, and is continuous between the two, by the isthmus
faucium, with the mouth cavity. Its floor is formed by the cranial end
of the larynx. At its caudal end it passes dorsally into the œsophagus,
while ventrally it communicates with the larynx. Its walls are muscular.
MUSCLES OF THE PHARYNX (Fig. 96).--=M. glossopharyngeus= (_i_).
_Origin._--Some fibres on the ventral and lateral part of the
genioglossus (_f_) leave that muscle near its caudal end. They form a
thin band of diverging fibres which pass outside of the cranial horn of
the hyoid. A similar sheet of fibres leaves the midventral part of the
styloglossus (_e_). The two sheets unite and the united muscle crosses
the hyoid, turns dorsad, and has its
_Insertion_ into the median dorsal raphe of the pharynx.
_Action._--Constrictor of the pharynx.
=M. constrictor pharyngis inferior= (_k_).--A thin sheet of muscle
covering the sides of the pharynx at its caudal end.
_Origin_ from the lateral surfaces of the thyroid and the cricoid
cartilages. The fibres pass dorsad and craniad, the cranial ones
covering the fibres of the middle constrictor (_j_).
_Insertion._--The median longitudinal raphe on the dorsum of the
pharynx. The caudal fibres are transverse and continuous with the
circular fibres of the œsophagus. The cranial fibres may pass as far as
the base of the sphenoid.
_Action._--Constrictor of the pharynx.
=M. constrictor pharyngis medius= (_j_).--A thin sheet which covers the
middle part of the lateral surface of the pharynx.
_Origin._--The ventral two pieces of the cranial horn and the whole of
the caudal horn of the hyoid. The fibres diverge, passing dorsad.
_Insertion_ into the median dorsal raphe of the pharynx. The cranial
fibres are inserted into the base of the sphenoid bone. The muscle
covers part of the stylopharyngeus (_l_) and the superior constrictor
(Fig. 66, _f_, page 112) and is partly covered by the glossopharyngeus
(Fig. 96, _i_).
_Action._--Constrictor of the pharynx.
=M. stylopharyngeus= (_l_).
_Origin_ from the tip of the mastoid process of the temporal bone and
from the inner surface of the cartilaginous piece between the
tympanohyal and the stylohyal bones. The parallel fibres form a flat
band which passes ventrocaudad over the outer surface of the constrictor
superior.
_Insertion._--The ventral fibres pass beneath the middle constrictor
(_j_) at its cranial border and, continuing toward the middle line of
the pharynx, gradually lose themselves among the fibres of the superior
constrictor. The dorsal fibres pass onto the outer surface of the middle
constrictor and are lost among its fibres.
_Action._--Constrictor of the pharynx.
=M. constrictor pharyngis superior= or =pterygopharyngeus= (Fig. 66,
_f_, page 112).--A flat, triangular sheet beneath the constrictor
medius.
_Origin._--The tip of the hamular process of the pterygoid bone. The
muscle passes caudad, the fibres diverging, and dips beneath the cranial
border of the constrictor medius.
_Insertion_ into the median dorsal raphe of the pharynx. The dorsal
fibres are inserted into the base of the sphenoid. The ventral fibres
pass lengthwise of the pharynx, closely connected with those of the
stylopharyngeus (Fig. 96, _l_), and finally reach the level of the
larynx.
_Action._--Constrictor of the pharynx.
3. =The Œsophagus.=--The œsophagus is a straight tube, dorsoventrally
flat when empty, which extends from the pharynx to the stomach. It has a
uniform diameter when moderately dilated of about one centimeter. It
lies dorsad of the trachea and against the longus colli muscles (Fig.
72, _g′_) covering the centra of the cervical vertebræ, until it reaches
the caudal end of the thyroid gland (Fig. 96, 6); then it passes to the
left and lies laterodorsad of the trachea until it reaches the
bifurcation of the trachea. It there returns to the median line, passes
gradually distad, separated from the vertebræ by the aorta, and finally
pierces the diaphragm about two centimeters from the dorsal body wall,
and enters the stomach. Its attachment to the diaphragm is loose enough
to permit of longitudinal motion. In passing through the thoracic cavity
it lies in the posterior mediastinum ventrad of the aorta. Its wall
consists of a muscular coat, a submucosa, and a mucosa, and its inner
surface presents many longitudinal folds. It has no serous covering, its
side walls being merely in contact with the halves of the mediastinal
septum.
4. =The Stomach. Ventriculus= (Fig. 97).--The stomach is the widest part
of the alimentary canal. It is a pear-shaped sac, the long axis of which
is curved nearly into a semicircle. The broad end of the sac lies to the
left and dorsad; here the stomach communicates with the œsophagus (_a_).
The narrowed end extends to the right and lies more ventrad than the
other end; it passes here into the duodenum (_g_). That portion of the
stomach which communicates with the œsophagus is known as the =cardiac=
end (_b_); the opposite is the =pyloric= end. Owing to the curved form
of the stomach above mentioned it is possible to distinguish a concave
and a convex side. The concave side is directed craniad and dextrad; it
is called the =lesser curvature= of the stomach (_c_). The longer convex
border is directed caudad and to the left; it is called the =greater
curvature= (_d_). The greater curvature extends to the left, next to the
œsophagus, into a prominent convexity known as the fundus (_e_) of the
stomach.
The stomach lies at the cranial end of the abdominal cavity, mostly to
the left of the middle line. Its cardiac end is in contact by its dorsal
surface with the dorsal, nearly horizontal, portion of the diaphragm. On
its ventral side the cardiac end does not touch the diaphragm, so that a
small part of the œsophagus passes here for a short distance into the
abdominal cavity, to join the stomach. The communication of œsophagus
and stomach is by a simple conical increase in size of the former. The
pyloric end of the stomach extends to the right of the middle line,
becoming constantly smaller; at its junction with the duodenum there is
a constriction which marks the position of the =pyloric valve= (_f_).
This valve is formed by a ring-like thickening of the circular
muscle-fibres of the alimentary canal, forming a sphincter muscle at the
junction of the stomach and duodenum and causing a projection of the
mucosa into the lumen of the canal. The ventral surface of the stomach
lies against the liver except when the stomach is much distended with
food, when the ventral surface comes to lie against the ventral
abdominal wall.
[Illustration: FIG. 97.--STOMACH, VENTRAL VIEW.
_a_, œsophagus; _b_, cardiac end of the stomach; _c_, lesser
curvature; _d_, greater curvature; _e_, fundus; _f_, pyloric valve;
_g_, part of duodenum.]
The stomach is supported by the =great omentum= and the =gastrohepatic=
ligament. It is connected with the duodenum by the =gastroduodenal=
ligament; with the spleen by the =gastrolienal= ligament.
The inner surface of the stomach presents longitudinal folds at its
pyloric end and along the greater curvature as far as the fundus. The
prominence of these depends on the degree of distension. Its walls are
composed of an external peritoneal layer, an internal mucous layer, and
an intervening muscular layer. This may be seen with the unaided eye in
sections of the wall.
5. =The Small Intestine. Intestinum tenue.=--The small intestine lies in
numerous coils which take up the greater part of the space in the
abdominal cavity. It has a length about three times that of the body of
the cat. It is usually considered as divided into three parts,
=duodenum=, =jejunum=, and =ileum=; these divisions are, however, not
clearly marked off. The small intestine is suspended by the =mesentery=
already described.
The =duodenum= is that part of the small intestine which follows the
stomach. At the pylorus (Fig. 97, _f_) the alimentary canal makes a
rather sharp turn so that the first part of the duodenum forms an angle
with the pyloric portion of the stomach, and extends caudad and slightly
toward the right, soon becoming directed almost entirely caudad and
lying along the right side. About eight or ten centimeters caudad of the
pylorus it makes a U-shaped bend, extending thus craniosinistrad for
four or five centimeters. Here it passes without definite limit into the
jejunum, the duodenum being considered to end at the next turn caudad.
The entire duodenum is about fourteen to sixteen centimeters in length.
Between the two limbs of the U-shaped bend formed by the duodenum, the
duodenal half of the pancreas is enclosed (Fig. 102, _a_).
The walls of the duodenum are composed of the serous (peritoneal)
investment, a muscular coat which is made up of an outer thin,
longitudinal layer of fibres and an inner thick, circular layer, a
submucous coat, and inside this a mucous coat. The mucosa is thrown up
into numerous delicate finger-like villi which give to it a velvety
appearance. On the dorsal wall of the duodenum, about three centimeters
distad of the pylorus, the mucosa presents a slight papilla, at the apex
of which is seen the oval opening of the =ampulla of Vater=. This is an
ovoid space in the wall of the duodenum. The space is encroached upon by
numerous folds of the walls. The common bile-duct and the pancreatic
duct open into it, the former extending from the bottom of the ampulla
nearly to its mouth, and the latter extending from the bottom about
half-way to the mouth. Two centimeters caudoventrad of the opening of
the ampulla of Vater is the opening of the accessory pancreatic duct. It
can usually be demonstrated only by passing a bristle into the duodenum
through an opening in the duct.
The =jejunum= is the part of the small intestine following the duodenum.
It is not separated from the part of the small intestine following it by
any sharp line. In man it constitutes two-fifths of the small intestine
exclusive of the duodenum, and is characterized by its emptiness after
death and by the absence from it of Peyer’s agminated glands (Peyer’s
patches).
The =ileum= is the portion of the small intestine between the jejunum
and colon. It lies suspended by its mesentery in numerous folds in the
caudal part of the abdominal cavity, separated from the ventral
abdominal wall only by the great omentum. It is of nearly uniform
diameter, but its caudal portion is thinner-walled than its cranial
portion. Its walls have a microscopic structure like that of the
duodenum and jejunum. On its inner surface and on the inner surface of
the jejunum are seen close-set villi, but these become rather sparser
toward the caudal end of the ileum and disappear about one centimeter
from the opening into the colon. Among the villi of the caudal end of
the ileum are numerous rounded elongations free from villi. These are
the solitary follicles or solitary glands (lymphatic) of the intestine.
These glands when aggregated together form the agminated glands or
patches of Peyer. The ileum passes at the caudal end into the colon, the
opening being guarded by the =ileocolic valve= (Fig. 99). This is formed
by a marked projection of the mucosa (_f_) and transverse muscle layer
(_e_) of the ileum into the colon. Its surface is free from villi.
6. =The Large Intestine. Intestinum crassum=.--The large intestine is
divided into =colon= and =rectum=. The =colon= or first part of the
large intestine lies against the dorsal body wall and is separated from
the ventral body wall by the folds of the ileum. It has a diameter about
three times that of the ileum. The opening of the ileum into it is on
its side between one and two centimeters from its cranial end (Fig. 98).
The blind pouch thus formed by the cranial end of the colon is the
=cæcum= (Fig. 98, _c_; Fig. 99, _a_). The cæcum ends in a slight conical
projection which may be considered as the rudiment of a vermiform
appendix. The colon lies at first on the right side and passes at first
craniad; then transversely to the left, then caudad, lying nearly in the
middle line and next to the dorsal abdominal wall. The colon may thus be
distinguished according to its direction into =ascending, transverse,=
and =descending= colon. At its caudal end the colon passes without sharp
limit into the rectum.
[Illustration: FIG. 98.--JUNCTION OF SMALL AND LARGE INTESTINE.