ORGANS OF THE ENERGIZING SYSTEM 85 



oxygefi- containing medium. Oxygen at a relatively high tension 

 in the 0-medium passes through the respiratory membrane and 

 through the walls of the capillary vessels to the O- carrier — where 

 oxygen is at a relatively low tension. CO 2, at a relatively high 

 tension in the fluid part of the blood passes through the capillary 

 walls and the respiratory membrane to the respiratory medium 

 (air or water) where it is at a relatively low tension. 



Thus the blood that goes to its termini in the tissues carries 

 oxygen as well as the proximate food substances. When it returns 

 from the termini it carries back CO 9, to be passed out into the 

 water or air. 



We do not know the precise physical details of the respiratory 

 gaseous interchanges — in spite of much research. Nevertheless, 

 it is without doubt that all the details of these processes are 

 strictly physical and chemical in the ordinary senses of these terms. 



31. ON ASSIMILATION 



Chemical analysis of the substances of the animal body shows 

 that these are roughly as follows : 



(/) The ubiquitous water of which all tissues contain, say, 

 from 30 to 80 or so per cent. 



{ii) The skeletal matrices : bone, shell, cartilage, chitin, 

 hydrated silica, sclero-proteins (in horn, feathers, etc.). 



{Hi) The structural ''protoplasm,'' that is, the substances of 

 the tissue-mechanisms — muscle substance, that of nerve, the 

 substance of gland cells, connective tissue, etc. Protoplasm is 

 not a single substance but a complex of proteins, lipins, etc. 



{iv) The reserves : fat in adipose tissues ; glycogen, or other 

 carbohydrate in the liver or muscles ; possibly the globulins and 

 albumens of the blood. 



It will be convenient (and adequate for the present purpose) 

 to consider the structural protoplasm, the blood and the reserves 

 as constituted by proteins, fats and carbohydrates. 



These substances are not necessarily the same in diflferent species 

 of animals. Thus there are the differences between the fats of 

 bacon, of beef and of mutton ; between the proteins of mutton, 

 lean beef, cod, peas, cheese, etc. In all these cases the protein 

 has the same general chemical structure, a complex of amino-acid 

 " building stones," but the specific proteins are recognizably 



