THE VASCULAR SYSTEM 319 



the circulation of the liquid medium, which is here termed 

 lymph and consists of plasma containing leucocytes alone. 



These two latter elements constantly escape from the blood 

 through the walls of the capillaries during the process of 

 feeding the tissues, and it is one of the functions of the lym- 

 phatic system to collect these by means of its smaller vessels 

 and eventually to return them to the blood. The other main 

 function of the lymphatic system is to aid in the extraction 

 of digested foods from the alimentary canal and convey them 

 also to the circulatory system. 



In no system of the body does the embryonic record tell 

 the story of the race development so completely as in the 

 case of the circulatory system, and although the change in 

 vertebrate history from water to air, replacing one set of 

 respiratory organs by another, has profoundly modified the 

 blood-vessels, yet even this change is repeated with great 

 fidelity in the individual life of each of the higher vertebrates. 

 This might be expected of the amphibians, in the most of 

 which the actual change of external environment is individually 

 experienced, yet a similar metamorphosis in the circulatory 

 system takes place in Sauropsida and Mammalia, although 

 it is confined to embryonic life. 



Since this is so, the best introduction to the history of the 

 circulatory system is that furnished by embryology, the early 

 part of which may be here given in the form of a general 

 sketch, which, although not intended to represent the details 

 of development in any one animal, or even of any one group, 

 yet is based rather more upon the development of the higher 

 vertebrates, since in these alone is the story complete. In 

 beginning this sketch a certain characteristic of nearly all 

 vertebrate embryos must be emphasized, since it is closely 

 connected with the circulatory system, especially in its earlier 

 stage, and that is, the extra-embryonal yolk-sac, which de- 

 velops a set of blood-vessels for the purpose of feeding the 

 embryo. 



In this is seen a probable reason why the history of this 

 system is retained in so much more perfect condition than 



