262 



THE VITAL FUNCTIONS. 



351) is placed at the beginning of these tubes, 

 where they arise from the ventricle. These valves 

 consist usually of three membranes, which have 

 the form of a crescent, and are capable of closing 

 the passage so accurately, that not a drop of 

 blood can pass between them.* 



In order to convey a more clear idea of the 

 course of the blood in the circulatory system, I 



have drawn the diagram, 

 Fig. 352, exhibiting the 

 general arrangement of 

 its component parts. The 

 main arterial trunk, or 

 Aorta (a), while proceed- 

 ing in its course, gives off 

 numerous branches (b), 

 which divide and subdi- 

 vide, till the ramifications 

 (p p) arrive at an extreme 

 degree of minuteness ; 

 and they are finally distributed to every organ, 

 and to the remotest extremities of the body. They 

 frequently, during their course, communicate with 

 one another, or atiastomose, as it is termed, by 

 collateral branches ; so as to provide against in- 



* In the artery of the Shark, and other cartilaginous fishes, 

 where the action of the vessel is very powerful, these valves are 

 much more numerous, and arranged in rovers, occupying several 

 parts of the artery. Additional valves are also met with in other 

 fishes at the branching of large arteries. 



