Structure —The cesophagus is composed a muscular and a mucous coat, with some con necting cellular tissue. The muscular coat the cesophagus, which is considerably thick : than that of any other portion of the alimen tary canal, consists of two distinct layers. TI external layer is composed of fibres arrang• in a longitudinal direction, and is twice t thickness of the internal layer, the fibres which surround the canal in a circular ma ner. The longitudinal fibres are regular! disposed around the cesophagus ; superior! they arise in the median line from the po. terior surface of the cricoid cartilage, an laterally on each side from the lower bord: of the inferior constrictor muscle of the ph rynx ; at the inferior extremity of the cesoplia gus they spread out and are continuous with the longitudinal fibres of the stornach. The circular fibres are a continuation of the pos terior part of the inferior constrictor muscle, but they are much more delicate than the fibres of this muscle. These fibres pass for the most part transversely ; the spiral arrangement which some anatomists have described does not ge nerally exist in the human subject. The co lour of the muscular fibres is pale red, less pale than those of the succeeding portions of the alimentary canal, and less deeply co loured than those of the pharynx. A micro scopic examination shews them to be com posed of both striped and unstriped fibres, mingled to an uncertain extent. " In some specimens from the human subject we have failed in detecting any striped fibres in the lower half of the cesophagus, either in the circular or longitudinal layer ; but, in other examples we have found them to within an inch of the stomach."* Mucous membrane.—The mucous membrane of the cesophagus is of a pale colour ; it pre sents a number of loncritudinal furrows, which are produced by a sli:flit folding of the mem brane during the partial contraction or ordi nary tonicity of the circular muscular fibres : the apparent laxity of the mucous membrane is no more than sufficient to allow of the dila tation of the canal which occurs during the process of deglutition. In addition to the longitudinal furrows there are some finer lines or wrinklings passing in various and indefinite directions, which are analogous to the fine grooved lines observed in the skin of various parts of the body. The mucous membrane is remarkable for its thickness ; the epithelium is so abundant as to be distinctly visible to the naked eye • it forms a thick layer similar to a cuticle, and terminates at the cardiac orifice of the stomach in a well-defined irregular hinged border. It is composed of the lamel liform or scaly variety of epithelium.t The mucous membrane is connected with the sub jacent muscular layer by the intervention of an abundant lax areolar tissue, which allows of a movement of these membranes upon each other during the repeated variations to which the diameter of the cesophagus is subject in the process of deglutition.
ffsophagealglands.—In the submucous areo lar tissue of the cesophag,-us are found a number of small glands. They may be felt through the mucous membrane, which they elevate here and there, as little circular or oval flattened granular bodies ; they are most numerous at the lower extremity of the tube. Their struc ture is the same as that of the buccal and duo denal glands. From the duct, which opens on the free surface of the mucous membrane, a few ramifications proceed and become em bedded in the submucous areolar tissue. The, branches are short and sacculated, having the appearance of small vesicles collected on a common stalk. The epithelium lining these glands is of the spheroidal variety4 Vessels and nerves.—The arteries distributed to the cesophagus are derived from several sources. In the neck they come from the in ferior thyroid artery ; in the chest some come directly from the aorta, others from the inter costals, and occasionally soine from the internal mammary arteries; in the abdomen, branches are derived from the coronaria ventriculi and from the phrenic arterie.' The veins corre sponding to these arteries,,empty themselves into the inferior thyroid, the superior cava, the azygos, internal mammary, coronaria ventriculi, and phrenic veins. The lymphatics open into
the glands which surround the cesophagus in considerable numbers. The nerves are derived chiefly from the pneumo-gastric. The recur rent branch of the pneumo-gastric in its course upwards sends numerous filaments to the ceso phagus. In the chest, as the trunks of the pneumo-gastric nerves lie on the cesophagus, each one sends 'off filaments which pass backwards,%encircling the tube, and meeting with branches from the opposite nerve. The plexus thus formed is called the plexus gulre ; it is joined by some filaments from the thoracic ganglia of the sympathetic.
Function.—The office of the cesophagus is to receive the aliment from the pharynx and to convey it into the stomach. This, the third and last stage of the process of deglutition, is un attended by sensation and uninfluenced by vo lition. The following is the mode in which the food is transmitted along the cesophagus. After being duly masticated and moistened in the mouth, it is received into the pharynx, and is thence propelled into the upper orifice of the cesophagus. The muscular fibres, both cir cular and longitudinal, of that part of the tube into which the food is propelled are at once stimulated to contract ; the mass is conse quently pushed onwards into the relaxed por tion of the tube immediately succeeding; the stimulus of contact with this part produces the same effect upon it as has already been pro duced upon that part of the tube which the food has just quitted, and the contraction of the first portion continuing at the time when that of the second portion is taking place, the substance is necessarily propelled onwards: it thus comes into contact with successive portions of the tube, and in each successive portion the same effect is produced, the contact of the sub stance exciting contraction, and the remaining contraction of the part which it has just quitted preventing regurgitation. These phenomena occur in a much less space of time than is occupied in their description, and the food is rapidly transmitted along the entire length of the canal. A notion may be formed of the rapidity with which these contractions are trans mitted along the cesophagus by observing the rapid vibrating movements in the neck of a horse while drinking. The secretion constantly poured out by the cesophageal glands_ has the effect of moistening and lubricating the interior of the tube, and thereby of facilitating the transmission of solid portions of food. The contractions of the cesophagus, which ordinarily commence at its pharyngeal and terminate at its cardiac extremity, sometimes take place in a reverse order, the direction of the movement depending on the part to which the stimulus is first applied. Dyspeptic persons are not un cominonly troubled with eructations of a liquid from the stomach, giving rise to what is fami liarly called heartburn; and in pyrosis, or water brash, the amount of liquid which suddenly enters the mouth is often very considerable. This inverted action of the cesophagus admits of a ready explanation. By the contraction of the muscular fibres of the stomach a portion of liquid is expelled into the lower extremity of the cesophagus; here it immediately excites contraction of the muscular fibres which sur round it, and being prevented from again en tering the stomach by the momentary continu ance of the same effort which has expelled it into the cesophagus, it must necessarily pass into the relaxed portion of the tube immedi ately above; and thus, by the contraction of successive portions of the tube, the liquid soon reaches the pharynx. In ruminants, the greater portion of the food is returned from the sto mach to the mouth by this inverted action of the cesophagus. • During the action of vomiting there is an inverted action of the cesophagus in addition to the propulsive effort arising from the contraction of the stomach and abdominal muscles.