ON THE DIGESTIVE FUNCTION, &c. 



12T 



digestive canal, the food on which they feed being already of their own 

 nature, and containing- a larg'er quantity of nourishment in a less bulk ; and 

 hence demanding a smaller proportion both of time and space to become fit 

 for use. In this respect man holds a medium between the two : his digestive 

 canal is less complex than that of most animals that feed on grass alone, and 

 more extensive than that of most animals that are confined to a diet of their 

 own kind. Man is hence omnivorous, and is capable of subsisting on an 

 aliment of either sort ; and from his digestive organs, as well as from various 

 others, is better qualified for every variety of soil and climate than any other 

 animal. , // 



Man, however, is by no means the only omnivorous animal in the world ; 

 for the great Author of nature is perpetually showing us that, though he ope- 

 rates by general laws, he is in every instance the lord and not the slave of 

 them. Hence, among quadrupeds, the swine, and among insects the ant, 

 possesses as omnivorous a power as man himself, and feeds equally on the 

 fleshy parts of animals, and on grain, and the sweet juices of vegetables. In 

 consequence of this omnivorous power in the ant, we may often make use of 

 him as a skilful anatomist ; for, by putting a dead frog, mouse, or other small 

 animal in a box perforated with holes, and placing it near an ant-hill, we shall 

 find it in a few days reduced to a perfect and exquisite skeleton, every atom 

 of the soft parts being separated and devoured. 



The solid materials of the food are first masticated and moistened in the 

 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 

 paste, which is called chyme ; and shortly afterward, by an additional pro- 

 cess, into a fluid for the most part of a milky appearance, denominated 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 line the whole of the interior coating of that part of the 

 intestinal tube into which the stomach immediately empties itself, and which 

 are perpetually waiting to imbibe its liquid contents. These vessels consti- 

 tute a distinct part of the lymphatic system ; they are called lacteals from the 

 usual milky appearance of the liquid they absorb and contain. They pro- 

 gressively anastomose or unite together, and at length terminate in one 

 common trunk, named the thoracic duct, which conveys the different streams 

 thus collected and aggregated 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 con- 

 traction of its fibres, and so far produces a mechanical resolution : secondly, 

 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 moist- 

 ened 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 duodenum, 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 concerning 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 consi- 

 derable size, and appears to produce a very striking effect on the blood itself, 

 by a removal of several of its prhiciples independently of its oflice as a 

 digestive organ. 



