96 



FIRST LESSONS IN ZOOLOGY 



partly separate from that out of the lungs. This separa- 

 tion allows the blood from the lungs to be sent to the 

 tissues without much mixing with the impure blood from 

 the tissues. In birds and mammals the separation of the 

 two halves of the ventricle is complete, the blood from 

 the lungs being sent out unmi.xed to the tissues and that 

 from the tissues returned without delay to the lungs. 

 Fig. 62 gives the plan of the circulation in the mammals. 



Fig. Go. — Diagram of heart of 

 amphibian; r.iz, right auricle; 

 I. a, left auricle; v, ventricle. 

 (After Ritzema-Bos.) 



Fig. 61. — Diagram of heart of 

 a reptile; r,a, right auricle; 

 /.(7, left auricle; "',z', ventricles. 

 (After Ritzema-Bos.) 



It shows how the blood is driven through the lungs by a 

 special pump, the right ventricle, which is devoted to 

 that purpose alone. It also makes clear how the blood is 

 made to pass from the left ventricle to all parts of the body 

 (fig- 63). It ma}' be asked how, since the blood remains 

 in vessels during circulation, the tissue-cells receive aii)'- 

 thing from it. The blood as such does not reach the tissue- 

 cells. These are surrounded by a liquid, called lymph, 

 which fills the spaces between them. The capillary 

 blood-vessels run through this liquid and may not actu- 

 ally touch the cells themselves at all, or at only a few 

 points. The walls of the capillaries being very thin, 

 however, the substances needed by the cells diffuse from 

 the blood through the walls into the liquid and thence to 

 the cells themselves. On the other hand, substances from 



