240 THE VITAL FUNCTIONS. 



of the ventricle ; so that the valve is always kept 

 within the cavity of" the latter. In the auricle, the 

 same purpose is answered by the oblique direction 

 in which the veins enter it. 



Tiie 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 another 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 the Mollusca, the heart acquires greater size, 

 compared with the other organs, and exerts a pro- 

 portionally greater influence as the prime mover in 

 the circulation. A beautiful gradation may be per- 

 ceived in the developement of this organ in the 

 several orders of this class ; the Bracliiopoda having 

 two aortic hearts, one placed upon each of the two 

 lateral trunks of the branchial veins ;t the Gas- 

 teropoda having a single heart, furnished with an 

 auricle ; and the Acephala being provided with a 

 heart, which has a single ventricle, but two auricles, 



* In the artery of the Shark, and other cartilaginpus fishes, where 

 the action 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. 



t This is also the case with the Area, which is one of the 

 Acephalous Mollusca. 



