Chap, v.] HEART OF VERTEBRATES. 201 



floated upwards by the pressure of the blood contained 

 in the ventricles, when acted on by the contraction of 

 the walls of these cavities. 



The peculiarities of the muscular tissue of the 

 heart of vertebrates are dealt with in works on human 

 or general physiology ; but it must be pointed out that 

 this tissue is remarkable for the possession of haemo- 

 globin ; that, under appropriate conditions of warmth 

 and moisture, the heart of a frog or a tortoise will, 

 after removal from the body, continue to beat auto- 

 matically for a number of hours ; and that minute 

 threads of the tissue from certain regions possess the 

 same peculiarity. 



In the Mammalia the muscular tissue of the heart 

 is supplied with proper blood-vessels (the coronary 

 arteries), which arise directly from the aorta, and 

 after branching elaborately, unite into the coronary 

 veins which open into the right auricle. In. some, 

 especially Ungulates, a bone, which in the ox may be 

 as much as an inch in length, is developed in the walls 

 of the heart. An analogous development obtains in 

 the penis, where a bone is sometimes present. 



The ventricular portion of the heart gives off 

 vessels which are known as the arteries ; in the least 

 modified Fishes, and in the Ganoids, the common trunk 

 (coiius or trillions arteriosws) is, like the venous 

 sinus, contractile, but in the bony fishes this con- 

 tractile power is altogether lost, and the bulbus 

 aortas as it is there called, becomes simpler in con- 

 struction, while the valves which prevent the blood 

 from flowing backwards are ordinarily reduced to 

 two ; the loss of the valves is clearly correlated with 

 the loss of contractility, for there is not in the walls 

 of this bulb any means by which the column of 

 blood can be compressed, and thereby tend to be driven 

 back into the ventricle. Where contractility is, on 

 the other hand, retained, we find three (dog-fish) or 



