THE CIECULATION OP THE BLOOD. 



EvEEY tissue, every cell, requiring constant nourishment, 

 some means must necessarily have been provided for the con- 

 veyance of the blood to all parts of the organism. We now 

 enter upon the consideration of the mechanisms by which this 

 is accomplished and the method of their regulation. 



Let us consider possible mechanisms, and then inquire into 

 their defects and the extent to which they are found embodied 

 in nature. 



That there must be a central pump of some kind is evident. 

 Assume that it is one-chambered, and with an outflow-pipe 

 which is continued to form an inflow-pipe. This might be pro- 

 vided with valves at the openings, by which energy would be 

 saved by the prevention of regurgitation. In such a system 

 things must go from bad to worse, as the tissues, by constantly 

 using up the prepared material of the blood, and adding to it 

 their waste products, would effect their own gradual starvation 

 and poisoning. 



It might be conceived, however, that waste at all events was 

 got rid of by the blood being conducted through some elimi- 

 nating organs ; and assume that one such at least is set aside 

 for respiratory work. If the blood in its course anywhere 

 passed through such organs, the end would be attained in some 

 degree ; but if the division of labor were considerable, we 

 should suppose that, gaseous interchange being so very impor- 

 tant as we tiave been led to see from the study of the chapters 

 on general biology, and on muscle, organs to accomplish this 

 work might receive the blood in due course and return it to the 

 central pump in a condition eminently fit from a respiratory 

 point of view. 



Such, however, would necessarily be associated with a more 

 complicated pump ; and, if this were so constructed as to pre- 

 vent the mixture of blood of diflferent degrees of functional 

 value, higher ends would be attained. 



