MODERN CONCEPTION OF PROTOPLASM. 31 



maintain the composition of the blood at its normal, 

 rejecting any stuffs that vary from that normal, either 

 qualitatively or quantitatively, doing this work according to 

 laws quite different from the simple ones of diffusion or 

 solubility : thus sugar and urea are about equally soluble, 

 and yet the sugar is kept in the body, while the urea is cast 

 out. Even substances as insoluble as resins are removed 

 from the blood by the living cells of the kidneys. 



A considerable quantity of water, and traces of salts, fats, 

 etc., leave the body by the skin, but its chief use is to pro- 

 tect, and to regulate the temperature by variations in the 

 size of its blood vessels. 



This completes our sketch (a) of the process by which 

 the food becomes available for the organism as fuel for the 

 maintenance of its life energies, and (/') of the removal 

 of the waste products which are formed as the ashes of life. 



There are indeed some organs which we have not men- 

 tioned, such as the spleen, which seems to be an area for 

 the multiplication of red blood corpuscles (fishes, newts, 

 embryo-mammals) or for the destruction of worn-out cor- 

 puscles (mammals), and the thyroid gland, which seems 

 to have to do with keeping the blood at a certain standard of 

 efficiency ; but what we have said is perhaps enough to con- 

 vey a general idea of the processes of life in a higher animal. 



In conclusion, it is perhaps useful to remark that when in the 

 course of further studies the student meets with organs which are called 

 by the same name as those found in man or in Mammals, as, for 

 example, the "liver" of the Molluscs, he must be careful not to sup- 

 pose that the function of such a "liver'" is the same as in Mammals, 

 for comparatively little investigation into the physiology of the lower 

 types of animal life has as yet been made. At the same time, he must 

 clearly recognise that the great internal activities are in a general way 

 the same in all animals ; thus respiration, whether accomplished by 

 skin, or gills, or air-tubes, or lungs, by help of the red pigment (haemo- 

 globin) of the blood, or of some pigment which is not red, or occurring 

 without the presence of any blood at all, always means that oxygen is 

 absorbed almost like a kind of food by the tissues, and that the car- 

 bon dioxide which re.sltsu from the oxidation of part of the material of 

 the tissues is removed. 



MODERN CONCEPTION OF PROTOPLASM. 



The activities of animals are ultimately due to physical 

 and chemical changes associated with the living matter or 



