AN ESSAY ON FAT AND MUSCLE. 
599 
the food of vegetables and animals is either altogether different 
in their substance, or passes before being assimilated into a new 
form, we must admit that the nutrition and growth of both depend 
on chemical agencies, although these operate under peculiar con- 
ditions, and are influenced by the unknown force which is termed 
the vis vitae, or vitality, so as to produce results that cannot be 
imitated by the chemist. The food of vegetables is derived from 
the crude and simple materials which they absorb from the air, 
the earth, and the waters. These, after being converted by the 
powers of vegetable assimilation into the substance of the plant, 
acquire the characteristic properties of organized products. Hence, 
plants can grow at the expense of the elements around, where no 
living substance ever previously existed ; while animals, on the 
contrary, can only exist upon matters previously organised either 
by plants or other animals. In their well-marked forms, no two 
things can be conceived to offer a stronger contrast than these 
great divisions of organized beings ; yet the naturalist cannot deter- 
mine, in the animated chain, where the one ends or the other 
begins; nor can the chemist detect, by his analyses, any greater 
differences in their constituent parts. 
Before, however, we can comprehend the manner in which 
these changes are effected in the living organism, we must make 
ourselves acquainted with some of the laws, vital and chemical, 
that regulate the metamorphoses of these materials, and the in- 
terchange of atoms occurring between the blood and the structures 
in the process of nutrition. Accordingly, we will first direct your 
attention to the process by which the aliment is received into the 
bodies of animals, and prepared to form a part of their fabric. 
When the food has entered the stomach, the gastric juice is poured 
out, and the whole is converted into a pulpy mass termed chyme. 
The process by which this step is produced in the assimilation of 
the food constitutes what is commonly called digestion. The 
next step occurs in the intestinal canal, where the chyme is united 
with the biliary and pancreatic secretions, and becomes con- 
verted into chyle. Brande found no essential difference in the 
chyle of graminivorous and carnivorous animals. Dr. Marcet 
imagines that the former is less abundant in albumen than the 
latter; and Liebig informs us that all the compounds of proteine, 
absorbed during the passage of the chyme through the intestines, 
take the form of albumen. The chyle is absorbed from the inner 
surface of the intestines by a set of vessels termed lacteals, which 
commence by very minute orifices in incalculable numbers, and unite 
successively into larger and larger vessels until they form trunks of 
considerable size, which empty their contents into a receptacle for 
that purpose, forming the entrance to the thoracic duct. In this 
