98 



FIRST LESSONS IN ZOOLOGY 



capacity of absorbing ox}-gen at the lungs and of giving 

 it up to the tissues. 



The blood of vertebrates and of 

 many invertebrates possesses a re- 

 markable property that should be 

 noticed, that is, of forming a jelly- 

 like mass called a clot when a vessel 

 is broken. In other words, it is 

 able to close the opening with a 

 solid plug made up of its own sub- 

 stances. 



In higher vertebrates there is a 

 very perfect regulation of the heart- 

 beat, and of the narrowing or enlarg- 

 ing of the small arteries by the 

 influence of the nervous system on 

 their muscular walls. By this means 

 and the peculiar structure of vein and 

 arteries, and the use of valves, the 

 flow of blood is nicely regulated to 



the needs of each part of the body during its activity. 



Fi 



64. — Lower ixTv- 

 Ic^ :lih1 fijdt of cat, 

 allowing a r t tr i e s 

 (iiuttc-(i lines), veins 

 (1)1. lek lines), and 

 nerves (thin lines). 

 (After Keij^hard and 

 Jennings.) 



