124 OF VITAL MOTION. 



supplies of blood. By this arrangement it may be 

 supposed that the fibre of the heart is placed in the 

 conditions most favourable to the exaltation of its 

 mobility, for in other muscles we see that this 

 faculty is directly proportionate to the free supply of 

 oxygenated blood. A chief cause, however, may be 

 supposed to be the intimate relation which exists 

 between the heart and the blood, a relation which is 

 rather what it might be imagined to be in a structure 

 intermediate between erectile tissue and muscle, than 

 in ordinary muscle. This is not only seen in the phe- 

 nomena which have been noticed as occurring in the 

 heart of a frog during action, but also in the peculiar 

 vascularity of any heart, when compared with the 

 other muscles of the same animal. As contributing to 

 the peculiarity of the action of the heart, we must 

 also mention the shortness of the vessels, and the 

 existence of the foramina thesbesii, all of which are 

 favourable to the speedy entrance and escape of blood ; 

 and finally, we must notice the great variations in 

 the quantities of blood contained in the walls and 

 cavities of the heart. Now, on comparing voluntary 

 with involuntary muscle, there is good reason to be- 

 lieve that the more abundant vascularity of the former 

 is a chief cause of the greater readiness and rapidity 

 of action ; and for the same reason, therefore, we may 

 suppose the heart to possess a greater mobility than 

 either; hence there is no great difficulty in supposing 



