The membranous part of the stomach ap pears to be peculiarly distensible, so as readily to admit of having its capacity greatly and suddenly increased, in order to contain the large quantity of solids and fluids that are occasionally received into it, while its mus cular fibres and nerves are possessed respec tively of a high degree of contractility and sensibility, by which they act powerfully on its contents, propelling them, when necessary, into the duodenum, and thus reducing the bulk of the stomach to its ordinary standard. Besides the mucous fluid which the inner sur face secretes, in common with all other mem branes of this description, the stomach is sup posed to possess certain glands, adapted for the formation of a specific fluid, termed the gastric juice, which acts an important part in the process of digestion ; but the presence of these glands has been rather inferred from their supposed necessity, than from any actual ob servation of their existence.* From the peculiar form and disposition of what have been termed the muscular coats of the stomach, they not only enable the organ to contract in its whole extent and in all direc tions, but they give to its individual parts the power of successively contracting and relaxing, so as to produce what has been termed its peristaltic or vermicular motion.t The effect produced appears to be, in the first instance, to form in the interior of the stomach a series of folds or furrows, and at the same time to agitate the alimentary mass, so as to bring every part of it, in its turn, within the in fluence of the gastric juice, while the whole of the mass is gradually carried forwards to wards the pylorus, and is in due time dis charged from that orifice. The muscular fibres of the stomach, like all those that are con nected with membranous expansions, forming what are termed muscular coats, are not under the control of the will.
In consequence of the great degree of vitality which the stomach possesses, a circumstance in which it is surpassed by scarcely any organ in the whole body, it is very plentifully provided with bloodvessels and with nerves. 1 he arteries, according to the ordinary construction of the sys tem, are furnished by tile contiguous large trunks, while the veins, in common with all those that be long to what are termed the chylopoietic viscera, terminate in the vena portte.* The nerves of the stomach are not only very numerous, but they are remarkable for the number of different sources whence they derive their origin. These are, in the first instance, threefold ; it is fur nished with a large quantity of ganglionic nerves, in common with all the neighbouring viscera ; it likewise receives nerves directly from the spinal cord, and unlike all the other parts of the body, except what are termed the organs of sense, it has a pair of cerebral nerves in a great degree appropriated to it. The specific uses of these different nerves are not certainly ascertained, and it would scarcely fall under the immediate object of this treatise to enter upon the consideration of this point; but we may observe, that no organ, in any part of the body, partakes more fully of what may be considered as the actions of the nervous system, or is more remarkably affected by its various changes, including not merely those of a physio logical nature, but such likewise as are con nected with the various mental impressions.t
The two extremities of the stomach, by which the food is received and discharged, are respec tively termed the cardia and the pylorus. Their structure, in many respects, differs from that of the other parts of the organ. The cardia is remarkable for the great proportion of nerves which are distributed over it, and as these are principally derived from the par vagum, or the eighth pair of cerebral nerves, we may under stand why this should be the most sensitive part of the stomach. The pylorus is remarkable for the mechanical disposition of its muscular fibres, which form an imperfect kind of sphinc ter, by which the food is detained in the cavity until it has experienced the chemical action of the gastric juice. And besides the functions which are actually possessed by this part, many imaginary and mysterious powers were ascribed to the pylorus by the older physiologists. The sensibility of the stomach was supposed to reside more especially in this extremity ; it was selected by some of the visionary philosophers of the sixteenth and seventeenth centuries as being the seat of the soul, and even some of the moderns ascribe to it a kind of intelligence or peculiar tact, by which it is enabled to select the part of the alimentary mass, which has been sufficiently prepared to enter the duodenum, while it prevents the remainder from passing through its orifice, and retains it for the purpose of being still farther elaborated./ On account of the form and position of the stomach it is sufficiently obvious, that a con siderable proportion of its contents must he, at all times, below the level of the pylorus. The food is hence prevented from passing too hastily out of the organ, while we may conclude that the transmission of the food is almost entirely effected by the contraction of its muscular fibres, aided probably by the diaphragm and the abdominal muscles, but scarcely in any degree by the mere action of gravity.* It must, however, be observed that the position of the stomach generally,with respect to the neighbour ing organs, as well as the relation of its different parts to each other, varies considerably according to its state of repletion ; when it is the most fully distended, its large arch, which previously was pendulous, is now pushed forwards and raised upwards, so as to be nearly on the same level with the pylorus.-f When the food leaves the stomach, it is re ceived by the intestinal canal, a long and winding tube, which varies much in its diameter and its form, in the different parts of its course, but which, both in its anatomical structure and in its physiological functions, bears a consider able resemblance to the stomach. It may be said, in the same manner, to consist of three essential parts, the membranous, the muscular, and the mucous, which respectively serve to give it its form, to enable it to propel its con tents, and to furnish the necessary secretions. With respect to the form of its individual parts, it has been divided, in the first instance, into the large and small intestines, a division which depends upon the comparative diameter of the two portions, while each of these has been sub divided into three parts, depending more upon their form and their position than upon their structure or functions.