CHAP. IX The Divided Labours of the Body 147 



the same side, and from this it is forced through the vessels 

 of the lungs, returning to the upper chamber of the left 

 side, and so to the lower chamber of the left side. 



The way in which the blood is able to nourish the 

 tissues is as follows: — The outgoing vessels — arteries — enter 

 each mass of tissue ; within it they break up into number- 

 less very small, very thin-walled vessels— capillaries ; the 

 blood oozes through these into the small spaces — lymph 

 spaces — that occur throughout the tissues ; adjacent to these 

 spaces are the cells, which take from the lymph — the fluid 

 that fills the small spaces and the vessels connected with 

 them — what they need, and cast into it their waste. The 

 lymph spaces open into lymph vessels, which, as has been 

 noted, join the blood-vessels. Oxygen and carbonic acid, 

 being gases, pass directly from the blood through the walls 

 of the vessels to the tissues, and from the tissues to the 

 blood. 



The Changes within the Cells. — In speaking of proto- 

 plasm an outline of the kind of knowledge that we possess 

 of the chemical changes that take place within the cells 

 has been given. We know little more than the sub- 

 stances that enter the cells and the substances that leave 

 them. 



Perhaps even this is too much to say ; more exactly, we 

 know the substances that enter the body by the mouth and 

 nose, and through the alimentary canal and lungs ; we 

 know the substances that leave the body through the 

 kidneys, and, in expiration, through the nose. A large 

 amount of water and traces of other matters leave the body 

 as perspiration ; but the chief use of sweating is probably 

 the regulation of the temperature of the body, and the skin 

 should not be thought of as an excreting organ in the same 

 way that the kidneys are. The undigested matter that 

 passes from the food-canal has never been within the 

 blood, and does not therefore concern us in this inquiry. 



But we know very little more than this ; the analysis of 

 the precise changes that any particular mass of tissue exerts 

 upon the blood — i.e. the differences that must exist between 

 the substances entering it and the substances leavin^^ it — is 



