CIRCULATION 169 



complex chemical substances, known as hormones, from their 

 specific points of origin in the various endocrine glands to the 

 particular tissue or organ where their regulatory influence is to be 

 effected. So the circulatory system is a distributing system which 

 not only maintains a suitable environment for the myriads of 

 cells of the body, but also, in cooperation with the nervous system, 

 unifies the organs into an organism. (Fig. 126.) 



Various stages in the development of a circulatory system can 

 be traced in the Invertebrates. In some it consists merely of a 

 single cavity or several connected cavities filled with a fluid con- 

 taining various types of cells, while in others more and more 

 of the spaces are replaced by definite tubes, or vessels, for the con- 

 duction of the fluid. With the establishment of closed vessels, the 

 contractions of various organs and the movements of the body as a 

 whole can no longer be entirely depended on for the movement of 

 the fluid, and accordingly, in certain regions, a muscular layer is 

 developed in the walls of the vessels, which by rhythmic pulsation 

 forces the fluid along. Thus, for example, in the Earthworm there 

 is a fluid (coelomic fluid) within the body cavity, which is forced 

 about by the movements of the worm and bathes most of the 

 internal organs; and also a system of vessels (vascular system), 

 a part of which contracts rhythmically and distributes the rlood 

 to the individual cells. (Figs. 59, 60.) 



In the Vertebrates circulation is effected by two systems, the 

 blood vascular and the lymphatic systems. The blood vascular 

 system consists of vessels which distribute the blood composed of 

 a liquid plasma, in which float various formed elements, chiefly 

 red and white cells. The lymphatic system comprises spaces, 

 channels, and vessels in the lower Vertebrates, but in the Mam- 

 mals, including Man, it is essentially a network of vessels so that 

 in higher animals the so-called closed circulatory system gradually 

 takes the ascendency over the predominately open circulatory sys- 

 tem of lower forms. The lymphatics carry lymph which consists 

 of a liquid plasma with white cells. Both systems are closely asso- 

 ciated, but the lymphatic plays a relatively passive role, so it is the 

 blood vascular system that one ordinarily has in mind when speak- 

 ing of 'the circulatory system.' (Figs. 7, 114, 115, 120, 125, 126.) 



The essential elements of the blood vascular system are, first, a 

 muscular organ for propulsion of the blood, the heart, which lies 

 near the mid- ventral line in the anterior part of the coelom; and, 



