10 



DIGESTION. 



wlnlothe veins, in common with all those thatbe- 

 luni; to what aiv termed the chylopoietic viscera, 

 tcTininate in thu vena portac.* 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.-)- 



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.J 



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 



Winslow, sect. viii. 2. 72. .7 ; Hallcr, El. 

 Phn.xix. t. 16. .20; Blumenbach, Inst. Physiol. 

 $ 3a6 ; Bell's Dissect, p. 19 . . 25. pi. 3, 4. 



t Winslow, ubi supra, 78, 9; Hallcr, xix. 1. 21 

 Blumenbach, 355; Bell's Anat. v. iv p 64- 

 Waller, Tab. nerv. No. 3, 4. 



1 Richerand, Physiol. 23. $ 111, 2. 



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. 



Hut although it may be supposed, that the 

 division of the tube into the great and small in- 

 testines refers to their difference of size alone, it 

 is to be observed that they perform very differ- 

 ent functions, and are subservient to very differ- 

 ent purposes in the animal ceconomy. It is in 

 the small intestines, and more especially in the 

 first portion of them, termed the duodenum, 

 that what must be considered as the most essen- 

 tial or specific part of the function of digestion 

 is effected, the formation of chyle, while it is 

 almost exclusively in the duodenum and the 

 other small intestines, the jejunum and the 

 ileum, that the chyle thus produced is taken up 

 by the lacteals, in order to be conveyed to 

 the thoracic duct, and finally deposited in the 

 bloodvessels. 



The use of the large intestines, and more es 

 pecially of the colon, which constitutes a con- 

 siderable proportion of the whole, appears to be 

 more of a mechanical nature, serving as a depo- 

 sit or reservoir, in which the residuary matter 

 is received and lodged, fora certain period, until 

 it is finally expelled from the system. The 

 division between the parts of the small intestines, 

 to which the names jejunum and ileum have 

 been applied, is entirely arbitrary, as they ap- 

 pear to be precisely similar to each other, both 

 in their structure and their functions, but the 

 case is very different with respect to the duode- 

 num, which in both these respects possesses a 

 clearly marked and distinctive character. Of 



* Ilaller. ubi supra, 2. .4. 

 t Blumcnbach, 353. 



