CIRCULATION IN THE VERTEBRATA. 



189 



The arteries themselves, especially the main trunk of the 

 aorta, as it issues from the heart, are muscular, and when 

 suddenly distended, contract upon their contents. It was 

 necessary, therefore, to provide means for preventing any 

 reflux of blood into the ventricle during their contraction; 

 and for this purpose a set of valves (v, Fig. 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 sys- 

 tem, I have drawn the diagram, 

 Fig. 352, exhibiting the gene- 

 ral arrangement of its compo- 

 nent parts. The main arterial 

 trunk, or Aorta (A,) while pro- 

 ceeding in its course, gives off 

 numerous branches, (B,) which 

 divide and subdivide, till the 

 ramifications (p) arrive at an 

 extreme degree of minuteness; 

 and they are finally distributed 

 to every organ, and to the re- 

 motest extremities of the body. 

 They frequently, during their course, communicate with 

 one another, or anastomose, as it is termed, by collateral 

 branches, so as to provide against interruptions to the circu- 

 lation, which might arise from accidental obstructions in any 

 particular branches of this extended system of canals. The 

 minutest vessels (p,) which, in incalculable numbers, pervade 



* In the artery of the shark, and other cartilaginous fishes, where the ac- 

 tion of the vessel is very powerful, these valves are much more numerous, 

 and arranged in rows, occupying several parts of the artery. Additional 

 valves are also met with in other fishes at the branching of large arteries. 



