36 THE FUNCTIONS OF ANIMALS 



excreted into the alimentary canal in the bile or by the 

 cells lining the large intestine. 



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 (b) of the removal of 

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



Some organs have not been mentioned, such as the 

 spleen, an accessory reservoir for blood, also an area 

 for the multiplication of red blood corpuscles (fishes, 

 newts, embryo-mammals) or for the destruction of worn- 

 out corpuscles (mammals) ; and the various ** endocrine " 

 glands, which make and pour into the blood specific 

 substances called hormones, whose function it is to regulate 

 the activity of cells in other parts of the body. Thus the 

 thyroid glands form thyroxin, a general stimulant of 

 metabolism. But what we have said is perhaps enough to 

 convey 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 " (^f the Molluscs, he must be careful not to 

 suppose 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 hfe 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 carbon 

 dioxide which results 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 

 protoplasm. This is a mere truism. We do not know 

 the nature of this living matter ; perhaps our most certain 

 knowledge of it is, that in our brains its activity is associated 

 with consciousness. 



