200 MANUAL OF ANIMAL BIOLOGY 



formed in great numbers in the blood, and they congregate at the 

 focus of infection and actually eat up, or phagocytize, the invading 

 bacteria. They are aided in this work by a particular type of 

 antibody, the opsonins, present in the plasma which makes the 

 bacteria palatable, as it were, to the leucocytes. 



Of course, it is the lymph which actually bathes the cells and 

 comes into immediate contact with the infected areas and the 

 invading organism. The lymph, it should be emphasized, contains 

 both antibodies and leucocytes and is a first line of defense against 

 invasion. But the lymph itself in an infected area may pick up 

 some of the parasites or other substances harmful to the other 

 tissues of the host. Such lymph must not be permitted to enter 

 the blood stream for general circulation until the foreign materials 

 have been rendered harmless. This is the function of the lymph 

 nodes which are present at strategic points in the lymphatic 

 system. The lymph slowly filters through them and the cells of 

 the nodes are usually able to render the lymph sterile before 

 passing it on to the blood stream. (W. f. 124.) 



Temperature control. The continuous and rapid flow of the 

 blood stream through all the tissues of the body serves to keep 

 all parts at a fairly uniform temperature. In general, however, the 

 blood picks up excess heat in the tissues, particularly muscle 

 tissue, where rapid metabolism is occurring, and it dissipates this 

 heat at the surface of the body through the skin. In the case of 

 the so-called cold-blooded animals, like the Frog and, in fact, all 

 the Vertebrates except Birds and Mammals, there is no constant 

 body temperature ; it varies according to the temperature of the 

 environment. Hence the vascular system is not so important in 

 connection with body temperature as it is in the warm-blooded 

 animals in which the temperature must not be permitted to vary 

 more than a slight amount. Here the vascular system plays an 

 all-important role under the direction of the nervous system, the 

 basic feature of which is a variation in the amount of blood sent 

 to the surface of the body. This is accomplished by changes in 

 the size of the skin vessels. If excess heat is being produced, a 

 greater proportion of the blood goes to the skin for cooling. If 

 the external environment is sufficiently cool, the condition of the 

 returning blood will be normal. On the other hand if the temper- 

 ature of the environment is high, the skin itself will be hot and no 

 cooling will result. In such a case the blood gives up excess water 



