126 



ON THE DIGESTIVE FUNCTION, &c/ 



gaseous, etherealized, and invisible forms, it is probably alike exempted from 

 the law of change ; while the Christian looks forward with holy hope to a 

 period when this exemption will be general, and extend to every part and to 

 every compound ; to a period in which there will be nevv heavens and a new 

 earth, and what is now corruptible will put on incorruption. 



At present, however, we can only contemplate matter, under every visible 

 form and modification, as perpetually changing ; as living, dying, and reviv- 

 ing ; decomposing into its primordial elements, and recombining into new 

 forms, and energies, and modes of existence. The germ becomes a seed, 

 the seed a sapling, the sapling a tree : the embryo becomes an infant, the 

 infant a youth, the youth a man ; and, having thus ascended the scale of ma- 

 turity, both instantly begin the downward path to decay; and, so far as 

 relates to the visible materials of which they consist, both at length moulder 

 into one common elementary mass, and furnish fresh fuel for fresh genera- 

 tions of animal or vegetable existence. So that all is in motion, all is striving 

 to burst the bonds of its present state ; not an atom is idle ; and the frugal 

 economy of nature makes one set of materials answer the purpose of many, 

 and moulds it into every diversified figure of being, and beauty, and happiness. 



But till the allotted term of existence has arrived, animals and vegetables 

 are rendered equally capable of counteracting the waste they are perpetually 

 sustaining in their individual frames ; and are wisely and benevolently en- 

 dowed with organs, whose immediate function it is to prepare a supply of 

 reformative and vital matter adequate to the general demand. 



Of this class of organs in plants we took a brief survey in our eighth 

 lecture ; and shall now proceed to notice the same class as it exists in ani- 

 mals, and which is generally distinguished by the name of the digestive system. 



There is, perhaps, no animal function that displays a larger diversity of 

 means by which it is performed than the present : and, perhaps, the only point 

 in which all animals agree, is in the possession of an internal canal or cavity 

 of some kind or other in which the food is digested ; an agreement which may 

 be regarded as one of the leading features by which the animal structure is 

 distinguished from the vegetable. 



Let us then, in the first place, trace this cavity as it exists in man and the 

 more perfect animals ; the organs which are supposed to be auxiliary to it, 

 and the powers by which it accomplishes its important trust. Let us next 

 observe the more curious deviations and substitutes that occur in classes that 

 are differently formed : and, lastly, let us attend to a few of the more singular 

 anomalies that are occasionally met with, and especially in animals that are 

 capable of subsisting on air or water alone, or of enduring very long absti- 

 nences or privations of food. 



The alimentary cavity in man extends from the mouth through the whole 

 range of the intestinal canal:* and hence its diflerent parts are of very dif- 

 ferent diameters. In the mouth, where it commences, it is wider; it con- 

 tracts in the esophagus or gullet ; then again widens to form the stomach, 

 and afterward again contracts into the tube of the intestines. This tube 

 itself is also of different diameters in different parts of its extent ; and it is 

 chiefly on this diversity of magnitude that anatomists have established its 

 divisions. Its general length is five or six times that of the man himself; 

 and in children not less than ten or twelve times, in consequence of their 

 diminutive stature. In som.e animals it is imperforate ; it is so occasionally 

 in birds, and fishes, and almost uniformly so in zoophytes. 



Generally speaking, the extent of the digestive cavity bears a relation to 

 ,the nature of the aliments by which the individual is designed to be nou- 

 rished. The less analogous these aliments are to the substance of the animal 

 they are to sustain, the longer they must remain in the body to undergo the 

 changes that are necessary to assimilate them. Hence the intestinal tube 

 of herbivorous animals is very long, and their stomach is extremely large, 

 and often double or triple ; while the carnivorous have a short and straiglit 



* Study of Med. ii. 2. 



