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ON THE DIGESTIVE FUNCTION, ETC, 



mouth, excepting in a few cases, in which it is swallowed whole, it is 

 then introduced into the stomach, and converted into an homogeneous 

 pulp or pfiste, which is called chyme ; and shortly afterwards, by an addi- 

 tional process, into a fluid for the most part of a milky appearance, denomi- 

 nated chyle ; in which state it is absorbed or drunk up voraciously by 

 thousands and tens of thousands of little mouths of very minute vessels, 

 which are not often found in the stomach, but hne the whole of the inte- 

 rior coating of that part of the intestinal tube into which the stomach im- 

 mediately empties itself, and which are perpetually waiting to imbibe its 

 liquid contents. These vessels constitute a distinct part of the lymphatic 

 system ; they are called lacteals from the usual milky appearance of the 

 liquid they absorb and contain. They progressively anastomose or unite 

 together, and at length terminate in one common trunk, named the tho- 

 racic duct, which conveys the different streams thus collected and aggre- 

 gated to the sanguineous system, to be still farther operated upon, and 

 elaborated by the action of the heart and the lungs. 



The means by which the food is broken down and rendered pultaceous 

 after being received into the stomach are various and complicated. In 

 the first place, the muscular tunic of the stomach acts upon it by a slight 

 contraction of its fibres, and so far produces a mechanical resolution : se- 

 condly, the high temperature maintained in the stomach by the quantity of 

 blood contained in the neighbouring viscera and sanguiferous vessels, 

 gives it the benefit of accumulated heat, and so far produces a concoctive 

 resolution ; and thirdly, the stomach itself secretes and pours forth from 

 the mouths of its minute arteries a very powerful solvent, which is by far 

 the chief agent in the process, and thus produces a chemical resolution. 

 In this manner the moistened and manducated food becomes converted 

 into the pasty mass we have already called chyme : and fourthly, there are 

 a variety of juices separated from the mass of the blood by distinct glands, 

 situated for this purpose in its vicinity, which are thrown into the duode- 

 num, or that part of the canal into which the stomach immediately opens, 

 by particular conduits, and in some way or other appear to contribute to 

 the common result, and to transform the chyme into chyle, but concern- 

 ing the immediate powers or modes of action of which we are in a 

 considerable degree of darkness. Of these glands the most remarkable 

 and the most general are the liver and the pancreas or sweet-bread ; the 

 first of which secretes the bile, and is always of a considerable size, and 

 appears to produce a very striking effect on the blood itself, by a removal 

 of several of its principles, independently of its office as a digestive organ. 



From this brief survey of the process of digestion it is obvious that the 

 stomach itself performs by far the principal part ; in some animals indeed 

 it appears to perform the whole ; and it is hence necessary that we examine 

 the general structure and power^of this organ with a little more minuteness. 



In man the stomach is situated on the left side of the midriff* ; in its 

 figure it resembles the pouch of a bag-pipe ; its left end is most capacious ; 

 its upper side is concave, its lower convex ; and the two orifices for receiv- 

 ing and discharging the food are both situated in the upper part. In its 

 substance it consists of three distinct coats or layers, the external and in- 

 ternal of which are membranous, and the middle muscular. The internal 

 coat, moreover, is fined with a villous or downy apparatus, and is extremely 

 convoluted or wrinkled ; the wrinkles increasing in size as the diameter 

 of the stomach contracts. 



From what I have already observed it must appear that the process of 



