96 GENERAL PHYSIOLOGY. 



tive process is accomplished, partly by means of the bloodvessels of the 

 stomach and small intestine, and partly by the agency of the special 

 absorbent vessels known as the lacteals. These, after passing through 

 the absorbent glands, which elaborate the fluid, conveyed through them, 

 at last end in the chief absorbent trunk, named the thoracic duct, 

 which then pours its contents into the great veins at the root of the neck. 

 The part of the dissolved nutrient matters which enters the absorbents 

 of the small intestine consists of an opaque white fluid, called the 

 chyle ; and the formation of this fluid is termed the process of chylifi- 

 cation. In this way, partly directly and partly indirectly, the nutri- 

 ent substances of the food, dissolved and modified by the digestive 

 processes, enter the bloodvessels, and renew the materials of the blood. 

 The unabsorbed residue of the food arid digestive juices, gradually 

 passes from the small into the large intestine, in which, by a sort of 

 secondary or continued digestive process, any remaining nutritive mat- 

 ter is almost entirely taken up from it. The final residue, including 

 certain products of decomposition, and other substances thrown off from 

 the system by the liver and the lining membrane of the intestines, 

 forms the solid excreta or egesta, which are removed from the body 

 by the process of defalcation. 



The blood, thus nourished by what is termed the primary process of 

 assimilation, is conveyed through every part of the body, by means of 

 the heart, the arteries, the capillaries, and the veins. It is propelled 

 from the heart through the arteries, passes from them into the capilla- 

 ries, and returns thence to the heart through the veins. Thus the 

 function of circulation is performed, the parts just named constituting 

 its organs. In the higher animals, and in man, the circulation is 

 double, or consists of two circular currents, each proceeding from the 

 heart, and returning to that organ again ; one, passing through the 

 body, is named the systemic, the other, through the lungs, the pulmo- 

 nary circulation. In the former, a pure or arterial blood proceeds 

 from the heart, whilst an impure or venous blood returns to it : in the 

 latter, the blood issuing from the heart is venous or impure, whilst it 

 returns arterialized or pure. 



We have now arrived at the point at which the waste of the organs 

 concerned in the animal functions of sensation, mental action, and 

 motion, may be repaired by the great and common function of nutri- 

 tion proper, nutritive secretion, or secondary assimilation. To accom- 

 plish this, new materials in a dissolved state, derived from the blood, 

 percolate through the fine walls of the capillary vessels, and constitute 

 what is called the nutritive plasma. From this common transparent 

 colorless fluid plasma, which moistens every tissue of the body, the 

 elementary tissues of each organ appropriate, by their assimilative 

 property, such materials as are needed for their renovation, or the 

 restoration of their wasted molecules ; and, under the influence of their 

 plastic property, deposit new material, molecule by molecule, in the 

 place of the disintegrated or wasted substance, so as to preserve un- 

 changed, the characteristic elementary structure of the tissue, and the 

 general form of the organ so nourished. The residual plasma passes, 

 it is supposed, together with the products of the wasted tissue, back 





