NUTRITION GENERALLY. 699 



or at least, the products of organization. They are, indeed, fluid 

 tissues or fluid parts of tissues, being the movable contents of vessels 

 or tubes formed by the coalescence of hollow elongated nucleated cells. 

 Like the more solid tissues, they are subject to nutritive waste and 

 renovation. In its widest sense nutrition includes, therefore, the re- 

 newal of the lymph and chyle, the renewal of the blood, and the 

 maintenance or renewal of the solid tissues and organs of the body. 



In each of these processes there is one common character, viz., that 

 the renewed fluid or solid assumes some portion of a nutrient pabu- 

 lum, different from itself, and converts it into a likeness of itself. 

 Hence the term assimilation is employed, to indicate the assumption 

 of necessary material into itself, by a fluid or solid tissue. The nutri- 

 tion of the chyle and blood, or the processes of chylification and san- 

 guification, are especially included under the term primary assimila- 

 tion, whilst the nutrition of the solid tissues and organs from the blood 

 is described as secondary assimilation. 



Nutrition of the Chyle. 



The composition of this fluid, its relations to the food, and the share 

 which the process of absorption has in its formation, have already 

 been described. The nutritive character of that part of the process 

 of chylification, which consists in the absorption of fatty matters, has 

 likewise been indicated. The fatty particles of the chyle within the 

 lacteals, certainly differ from the emulsionized fat found on the surface 

 of the intestinal mucous membrane, and the albuminose of the in- 

 testinal canal appears as albumen and fibrin in the ehyle. Accord- 

 ingly, these last-named substances, and perhaps also the fat, undergo 

 true assimilative changes, and not a mere absorption, as they enter the 

 chyliferous vessels. Moreover, the corpuscles which appear, as the 

 fluid advances through the lymphatic plexuses and glands, afford dis- 

 tinct evidence of evolution, organization, and, therefore, of assimila- 

 tive power. The columnar epithelial cells upon the villi, and also the 

 cells of the absorbent vessels and the alveolar spaces of the lymphatic 

 glands, which are themselves developed by the coalescence of hollow 

 nucleated cells, the parietes of which continue to be endowed with 

 special properties of an elective and, therefore, assimilative nature, 

 participate in this process. If, as previously alluded to (p. 625), the 

 process be regarded as secernent, the movable contents of the lacteals 

 form a secretion, and the entire lymphatic system constitutes a large 

 gland, the secreting tubuli of which begin either amidst the tissues of 

 the body, as lymphatic plexuses, or in the villi of the small intestine, 

 as the commencements of the lacteals ; whereas the terminal ducts, 

 of which the chief one is the thoracic duct, open into the great veins 

 at the root of the neck. The vascularity of the septa within the 

 lymphatic glands, favors the near proximity of the fresh chyle or 

 lymph to the capillary bloodvessels ; in this way, these fluids may be 

 inspissated by venous absorption, and, moreover, by the action of the 

 arterial blood, the fibrinous matter may acquire the increase .which is 

 noticed beyond the lymphatic glands. 



