26 THE FUNCTIONS OF ANIMALS. 



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 {b) 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 blood corpuscles, 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 convey a general idea of the processes of 

 life in a higher animal. 



In conclusion, it is periiaps 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 suppose that the 

 function of such a " liver " is the sarne 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 (haemoglobin) 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 carbonic acid gas 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 ; in fact, our most certain know- 

 ledge of it is that in our brains its activity is expressed as 

 thought. 



When more is known in regard to the chemistry and 

 physics of living matter, it may be possible to bring vital 

 phenomena more into a line with the changes which are 

 observed in inorganic things. At present, however, it is 

 idle to deny that vital phenomena are things apart. Not 

 even the simplest of them can be e.xplained in terms of 

 chemistry and physics. Even the passage of digested food 

 from the gut to the blood vessels is more than ordinary 



