CHAPTER VIII 

 THE VASCULAR SYSTEM 



"However, if we consider that all the characteristics 

 which have been cited are only differences in degree 

 of structure, may we not suppose that this special 

 condition of organization of man has been gradually 

 acquired at the close of a long period of time, with 

 the aid of circumstances which have proved favorable? 

 What a subject for reflection for those who have the 

 courage to enter into it ! " 



LAMARCK in Recherches sur I' Organization 

 des corps vivans. 1802. Transl. Packard, 

 1901. 



A VASCULAR system of some sort occurs in all coelomate 

 animals, except in some reduced parasitic forms, and consists 

 essentially of a cavity, or series of connected cavities, in which 

 a fluid circulates, containing- detached cells of one or more 

 kinds. Both fluid and cells are concerned in metabolism 

 and act as carriers of material both to and from the various 

 tissues. In many Metazoa, especially the smaller and less 

 highly organized omes, the system is lacunar, and the circu- 

 lating medium, here often termed the peri-visceral fluid, occu- 

 pies everywhere the irregular spaces between the organs, and 

 its circulation is furthered by the movements of these latter 

 and of the entire body; in other cases the lacunar system be- 

 comes reinforced, or largely replaced, by the formation of 

 definite channels in the form of branching tubes, through 

 which the fluid circulates. Such a circulation is said to be 

 closed, in distinction from the lacunar or open type first 

 mentioned, and in such a system, deprived as it is of the pro- 

 pelling power insured by the movement of external parts, de- 

 pendence must be placed upon some intrinsic force within the 

 vascular system itself, and thus there arise pulsating vessels, 



317 



