254 



THE MECHANICS OF THE HEART 



as valve (A) is firmly closed at this time, a definite direction of 

 flow is now imparted to the fluid. It leaves the heart (H) through 

 the arterial orifice . (A) and cannot return to this organ until it has 

 traversed the entire tube (C) . 



The channel which conveys the blood away from the heart is 

 known as an artery, while the one returning the blood to this organ is 

 called a vein.^ In a true circulatory system these two divisions are 

 joined by a multitude of fine tubules, designated as capillaries, so that 



the- entire vascular system is really com- 

 posed of three parts, namely of arteries, 

 capillaries, and veins. In accordance with 

 certain structural pecuHarities, these chan- 

 nels may be subdivided further so that in 

 final analysis the circulatory system con- 

 sists of arteries, arterioles, arterial capil- 

 laries, capillaries proper, venous capillaries, 

 venules, and veins. The central arterial 

 tube is commonly spoken of as the aorta, 

 and the central collecting channel as the 

 vena cava. 



The Circulatory System in the Lower 

 Animals. — In the lowest forms the nutri- 

 tion of the outlying colonies of cells is 

 effected by progressive and oscillatory 

 streams which are brought into existence 

 by differences in pressure as well as by 

 the processes of diffusion and osmosis. In 

 the highest animals, on the other hand, 

 these simple movements give way eventu- 

 ally to a complex roundabout motion of 

 the body fluid, but this end is not attained 

 until the circulatory mechanism has passed 

 through several intermediary stages of de- 

 velopment. In order to be able to follow these changes more closely, 

 it seems advisable to initiate this discussion with a study of the con- 

 ditions existing in such forms as the sponges which may be said to 

 possess a circulation of the most elementary kind. We find here 

 that the water enters through numerous pores of the derma and is 

 then returned to the surrounding medium by way of the central canal 

 and the osculum. The power necessary to produce this flow is fur- 

 nished by the cilia with which the aforesaid passage is beset. The 

 higher ccelenterates are in possession of an alimentary canal, the 

 smaller recesses of which extend far into the substance of their 

 bodies. In this way, these saccular extensions are enabled to serve 



1 For this reason, the pulmonary artery is known as an artery, although it 

 contains venous blood, and the pulmonary vein as a vein, in spite of the fact that 

 it contains freshly aerated blood. 



Fig. 123. — Schema op Simple 

 clbculatoky system. 

 I, phase of contraction; II, 

 phase of relaxation of heart; A 

 and B, valves guarding cardiac 

 orifices; D, arteries; C, capil- 

 laries; E, veins. 



