ASSIMILATION 91 



Explanation of Fig. 60 



By means of blood and lymph vessels nutrients and oxygen are distributed to all cells 

 of the body, while excretions and endocrines are collected and transported from place to 

 place. The arrows indicate the direction of flow of the blood. 



A6. v., abdominal vein; .4 u, right auricle, Aft', left auricle; fi.Z)., bile duct; 5/, urinary 

 bladder; C.A., carotid artery; C-M., ccehaco-mesenteric artery; Cp.D.T., capillaries of 

 digestive tract; Cp.F-L, capillaries of fore-leg; Cp.Hd., capillaries of head; Cp.H-L, capil- 

 laries of hind-leg; Cp.A'ci., capillaries of kidney; C^^.SA;., capillaries of skin; Cu. A., cutaneous 

 artery; C.F., cutaneous vein; £>. A., dorsal aorta; iJ.r., digestive tract; F.V., femoral vein; 

 G, gonad; Gl, glottis; Glm, glomerulus; H.A., hypogastric artery; H.P.V., hepatic portal 

 vein; Hp.V., hepatic vein; I. A., iliac artery; J.V., jugular vein; L.H., lymph heart; 

 Lg, lung: Lib, liver; L.S., lymph sinus; L.V., lymph vessel; P. A., pulmonary artery; 

 Pa, pancreas; P-c.A., pulmo-cutaneous artery; Pcv, pre-caval vein; P-c.V., post-caval 

 vein; P.O., pancreatic duct; Pel.V., pelvic vein; P. F., pulmonary vein; 7Z. P. F., renal portal 

 vein; S, sinus venosus; S.A.. systemic artery; Sb.A., subclavian artery; Sb.V., sub- 

 clavian vein; Sk, skin; Sk.Gl., skin bland; Sp, spleen; T.A., truncus arteriosus; Ur, 

 ureter; F, ventricle. (Redrawn with modifications from Parker and Parker, " Practical 

 Zoology," copyright, 1916, by Macmillan and Co., Ltd., reprinted by permission.) 



later section (p. 101). Fat, also, may be stored in certain regions 

 of the body, such as the subcutaneous fascia, a layer between the 

 skin and muscles; in the mesentery; between the muscles; and, in an 

 animal like the frog, in special organs, the fat-bodies. As is the 

 case with stored carbohydrate, this fat can pass back into the blood 

 for distribution in case of cellular need. So far as we know, amino 

 acids are not stored in any part of the body but are taken from the 

 blood, as required, by all cells. 



Because of the ingestion of food, its digestion, absorption, and 

 distribution, the protoplasm of all cells of the body receives a 

 supply of the materials that are necessary for its metabolism. 

 The substances brought to the cells are synthesized under the 

 influence of cellular enz>Tnes, to form the constituents characteris- 

 tic of the particular protoplasm in which the synthesis occur.'-. 

 Then, finally, we have assimilation of the foods that enter the 

 body by way of the digestive tract. 



Respiration. — The term respiration has been widely used to cover 

 the so-called gaseous metaboUsm of an organism. It seems unnec- 

 essary, and perhaps unwise, to try to separate the metabolism of 

 oxygen from that of other substances, since all metabolic reactions 

 are so closely inter-related. If food is to be defined as any sub- 

 stance necessary for the normal functioning of the organism, oxygen 

 becomes a food, just as water and mineral salts are foods. Respi- 

 ration, then, is used here as a term covering the dehvery of oxygen 

 to the protoplasmic system. 



As was pointed out in the discussion of the structure of verte- 

 brates, respiratory systems are of two types, depending upon 



