THE walls of Compartment I are composed partly of bones and partly of voluntary muscles. The roof is made up of bones (maxillae and palate) which are immovable. The bone (mandible) entering into part of the side walls is hinged at the temporo-mandibular joints and may be moved in various directions. The side walls are completed by muscles acting on the mandible, cheeks, and lips. The floor consists largely of the tongue, with its foundation of intrinsic muscles that can alter the shape of the organ, and several extrinsic muscles that can move the tongue freely in many directions.
The isthmus of the fauces, which leads into Compartment II, is a ring of voluntary muscles ; the upper part is constituted by the soft palate, the lower by the base of the tongue, and the sides by the pillars of the fauces.
Compartment II consists of an upper portion, the pharynx, and a lower, the oesophagus. The pharynx has a basis of fibrous tissue covered by voluntary muscles. It is attached to bony structures in the neck and is therefore a semi-rigid tube which is permanently open. The tube of the oesophagus is directly continuous with that of the pharynx.
The wall of the upper third of the oesophagus is composed of striated muscles, but in the distal two-thirds these are replaced by involuntary muscles. It has no bony connexions, but strands of unstriped muscle and elastic tissue anchor it loosely to neighbouring structures. Except during the passage of foodstuffs the tube is usually closed.
The oesophagus passes through the diaphragm directly into the stomach; its muscular wall merges into that of Compartment III which is entirely involuntary.
The shape of the fully-developed Compartment III is defined by th e arrangement o f its muscle-fibres; th e disposition o f these is determined b y th e embryological history of this part of the alimentary tube.
Compartment III consists anatomically o f tw o portions, namely, th e stomach and a part of the duodenum, separated transversely b y a well-defined fibromuscular ring—the pyloric sphincter (Fig. 3 ) .
The stomach is subdivided longitudinally into: (a) A tubular part that starts at the cardiac end of the stomach, forms th e lesser curvature, a n d expands to take part in th e formation o f th e pyloric vestibule (o r antrum) a n d th e pyloric canal; a n d (b) A saccular part that forms the fundus and body of the stomach and includes the greater curvature; it merges gradually into th e pyloric portions.
The muscular tunic of the stomach diners somewhat from that o f th e fundamental pattern. T h e outer longitudinal coat o f th e oesophagus is continued down over th e tubular and saccular parts o f the stomach to th e neighbourhood o f the pyloric ring. Here the majority of the fibres end by turning into the pyloric valve; the remainder are continued into the wall of the duodenum.
The inner circular coat of the oesophagus is continued into a circular coat that surrounds both parts of the stomach; it, however, ends abruptly at the pyloric ring.
A third coat, internal to these, begins at the cardiac end of the stomach and gradually fans out to end amongst the circular fibres as it reaches the pyloric vestibule. From its disposition it appears to act as a supporting sling for the pyloric end, but at the same time it tends to separate roughly the tubular and saccular parts of the stomach.
The first half of the duodenum extends from the pylorus to the region where the bile-duct enters the duodenum; the first inch (approximately) of it forms the so-called duodenal cap. The muscular tunic of the duodenal cap is devoid of circular fibres and of muscularis mucosae. Beyond this the wall of the duodenum gradually assumes the fundamental pattern.
The stomach and the duodenal cap are suspended freely in the abdominal cavity; the rest of the duodenum is tied down by peritoneum and connective tissue to the posterior abdominal wall.
Compartment IV is divisible into: (a) A proximal part beginning about the middle of the duodenum and ending at the caeco-colic junction; and (b) A distal part that extends from this junction to the middle of the transverse colon. The proximal part at its commencement is a con- tinuation of the duodenum down to the duodenojejunal
junction; like the greater part of the rest of the duodenum, it is bound to the posterior abdominal wall by peritoneum and connective issue.
The next portion of the proximal part of Compartment IV consists mainly of the long stretch of the coils of jejunum and ileum; it is suspended from the posterior abdominal wall by the mesentery proper and within the limits of this is freely movable in the abdominal cavity. The muscular tunic of the proximal part of Compartment IV down to the ileocolic junction is of the fundamental pattern, namely, an outer longitudinal and an inner circular layer.
The terminal portion of the proximal part includes the ileocaecal region and the caecum. The ileocaecal region is anchored to the posterior abdominal wall. Here the circular fibres of the muscular tunic of the ileum are continuous with those of the caecum. Fibres from both run out to the lips of the ileocaecal valve, where they form a kind of sling in which end some longitudinal fibres of the ileum.
The muscular tunic of the caecum consists of circular and longitudinal fibres. Some of the longitudinal fibres are gathered into three strong narrow bands or taeniae; these can be differentiated first at the opening of the appendix, and thence are continued into the wall of the colon.
The caecum is completely covered by peritoneum, but the proximal part of its appendix is partly tied down by a short meso-appendix to the mesentery proper.
The caecum and appendix, therefore, are mobile to a limited extent.
The distal part of Compartment IV includes the ascending colon, the right colic (hepatic) flexure, and the right portion of the transverse colon. The ascending colon is usually fixed by peritoneum and connective tissue to the posterior abdo- minal wall, but the transverse colon enjoys a limited degree of mobility.
A feature of the muscular tunic of the distal part of Com- partment IV is the concentration of the longitudinal fibres into the three taeniae that begin at the base of the appendix. These bands when tonically contracted are shorter than the corres- ponding portion of the tube itself, with the result that this part of the alimentary tube assumes a sacculated form; a series of circular folds or shelves consisting of almost the whole thickness of the wall is projected into the interior of the tube. This structural arrangement differs from that seen in the interior of the small intestine, where the circular plicae involve only the mucous membrane. If the taeniae of the great intestine be cut the saccules disappear.
Compartment V begins about the middle of the transverse colon and ends at the anal canal. It includes the left part of the transverse colon, the left colic (splenic) flexure, the descending colon, the iliac colon, the pelvic colon, and the rectum; the iliac colon and pelvic colon together constitute the sigmoid colon. The part of the transverse colon that enters into the formation of Compartment V has a limited mobility, swinging, as it does, on the transverse mesocolon. The left colic flexure is anchored to the diaphragm. The descending colon and the iliac colon are usually bound to the posterior abdominal wall by peritoneum and connective tissue. The pelvic colon is mobile on account of its meso- colon. In practically its whole length the rectum is tied to the posterior wall of the pelvis by peritoneum and connec- tive tissue.
The characteristic arrangement, already described, of the muscular structure of the first part of the great intestine— namely, the three longitudinal taeniae—is continued along the remainder of the great intestine as far as the end of the pelvic colon; there is a notable increase in the thickness of the taeniae of the descending and pelvic parts of the colon.
In the rectum some modifications are introduced. In place of the three taeniae two strong longitudinal bands may be demonstrated, one on the front, and one on the back of the tube; in addition, at the lower end of the rectum volun- tary muscular fibres from the pelvic diaphragm join and intermingle with the involuntary muscles. The anterior and posterior longitudinal bands of the rectal wall, like the taeniae of the rest of the great intestine, are shorter than the tube itself—in this case markedly so. As a result the rectum exhibits a series of three lateral inflexions with corresponding shelves—the rectal valves—projecting into the interior of the tube. This arrangement permits of lateral expansions of the distended rectum without a corresponding elongation of the tube; at the same time, the infolded shelves serve to support the weight of the contained faeces.
The rectum passes through the pelvic diaphragm and is continued into the anal canal {Compartment VI). The in- voluntary muscular fibres of the anal canal, particularly those of the circular layer, are very thick and strong, and form the internal sphincter ani. Numerous voluntary muscular fibres from the pelvic diaphragm join in the forma- tion of the wall.
The terminal aperture (anus) is closed by striated muscular fibres—the external sphincter ani.
The Sphincters of the Alimentary Tube.—In Compart- ment I the contents are under voluntary control and, at will, may be either rejected or passed into Compartment II; the faucial ring of voluntary muscle controls the entrance to Compartment II. When, however, the food mixture passes through the pharynx and enters the oesophagus it is no longer under control of the will and under normal conditions is forced gradually onwards towards the terminal exit of the alimentary tube. Near the end of the alimentary tube a strong sphincter—the recto-anal—guards the outlet. This sphincter consists of mixed voluntary and involuntary fibres;
its effectiveness is substantially aided by the lateral pressure exerted by the levatores ani muscles.
In the rest of the alimentary tube involuntary muscle- fibres constitute the wall of the tube. Their regular arrange- ment is sharply interrupted at two sites by sphincters, namely, the pyloric valve and ileocaecal valve.
These four sphincters (faucial, pyloric, ileocaecal, and recto- anal) are recognizable macroscopically and microscopically, and in military parlance are the four great bastions of defence of the alimentary tube. In addition to them, however, there are other regions—zones of elastic defence—not so strictly delimited and probably occupying some length of the tube. They are not easily demonstrable anatomically, but are capable of definition by inference and occasionally radiographically. The chief of them are: (i) The sphincter at the lower end of the oesophagus—this junction is strength- ened by fibres derived from the diaphragm and by the anatomical disposition of the cesophageogastric aperture;
(2) A zone in the proximal part of the duodenum; (3) A zone at the junction of the caecum and ascending colon; (4) A zone in the middle of the transverse colon; (5) A zone at the junction of the pelvic colon with the rectum, the pelvi- rectal sphincter—this is the best defined of the group. These zones of elastic defence are situated in more or less fixed regions. Other more fleeting zones may appear in the form of constrictions (spasms) of the bowelwall when occasion arises.
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