146 The Study of Animal Life PART n 



but the waste nitrogenous matter is altered and then sent 

 on in the blood stream to the kidneys. 



The cells of the kidneys take this stuff, which was pre- 

 pared in the liver, and other waste nitrogenous products 

 out of the blood, pass them and a certain amount of water 

 along to the urinary bladder, which empties itself from time 

 to time. 



Respiration. Breathing consists of two distinct acts, 

 inspiration and expiration. During an inspiration air is 

 drawn into lungs. Thence the oxygen passes by diffusion, 

 modified by the fact that the essential membrane is a living 

 one, into the blood. There it enters into a loose combina- 

 tion with haemoglobin, the red colouring matter of the 

 blood cells, and is thus carried to the cells of the tissues to 

 be absorbed into their living matter. During an expiration 

 we breathe out air which has less oxygen and more carbonic 

 acid gas than normal air. The carbonic acid is a waste- 

 product formed by the cells of the body ; it first enters the 

 blood, is then carried to the lungs, and leaves the blood- 

 vessels by a process of diffusion similar to that by which 

 the oxygen entered. The close association of these two 

 processes is simply due to the fact that an organ fitted for 

 the diffusion of one gas in one direction will do for the 

 diffusion of all gases in any direction. 



Circulation. The blood is maintained in a healthy state 

 by the processes we have described. By the active con- 

 traction of the heart it is pumped round and round the 

 body, continually carrying with it fresh food to the tissues, 

 and carrying away with it the waste matter cast out of the 

 tissues. All the blood-vessels, except the very smallest, 

 have muscular walls. The heart is a large hollow mass of 

 muscles, is a part of a pair of large blood-vessels that have 

 been bent upon themselves, and arranged so as to form 

 four separate chambers, two upper and two lower, an upper 

 and a lower opening directly into one another on each side. 

 By the contraction of the lower chamber of the left pair the 

 blood is forced through all the vessels of the body ; these 

 collect and empty the blood into the upper chamber of the 

 right pair ; from this it passes into the lower chamber on 



