OK NUTRITION. 345 



distensions and pockets like so many lateral purses, the digestible 

 matter is imbibed by the parietes of the cavity, and so transformed 

 into the substance of the body. We find this first and most simple 

 mode of nutrition in the lowest animals, the Infusoria, the Polypi, the 

 Acalephte, and many of the intestinal worms. 



The digestion of the food can only be perfectly accomplished when 

 it has been previously adapted thereto by the secretions of peculiar 

 organs, which, as it were, kill and decompose it. Where such 

 auxiliary organs present themselves we find the cavity of the stomach 

 more complex, longer, and tubular, and making several convolutions in 

 the body. The first of the secreting organs that is added to the 

 digesting cavity, which we may henceforth call the intestinal canal, is 

 the liver, which is a glandular body that pours its secretion into the 

 anterior half of the intestine beyond the stomach, and which thereby 

 renders the chyme fit for absorption. The second secreting organs are 

 the salivary glands : they first present themselves in such animals which 

 take hard food, and by their secretion cause the transformation of the 

 coarse materials into a uniformly fluid pap. We find upon this grade 

 of the development of the digestive apparatus the muscles, snails, 

 Crustacea, Arachnids, Myriapodes, and insects. Many of them want 

 the salivary glands; many have a multilobed liver, as the snails; others 

 have a small one, in the form of tubular canals. The deficiency of an 

 anus is a rarity in this grade of organisation, but we however find it 

 among insects. 



Upon the third and last grade we observe not only the preceding 

 secreting organs both more perfect and numerous, but other new ones 

 present themselves, some of which pour fluids into the intestine, as the 

 pancreas; and others rectify the absorbed chyle, as the milt and kidneys ; 

 of the last, however, we observe occasional prefigurations in the snails 

 and insects. This most perfect development of the digestive apparatus 

 is found in the Fertebrnta. 



216. 



It does not suffice that the digestive organ should thus become by 

 degrees more perfect, thereby facilitating the separation of the nutritive 

 matter, but the imbibed and decomposed chyle must be subjected to 

 another change before it can be transformed into the organic mass. This 

 change is produced by means of respiration, a function which consists 

 in adding to the nutriment a new substance present in the atmosphere, 



