130 PHYSIOLOGY. 



Of the Second Division of Hearts. — This is a mixture between the 

 first and third, by which means it is more imperfect and much less 

 distinct than either. This heart consists of two distinct cavities, and of 

 two others which are not so perfectly distinct, and which act only as 

 one cavity. The two distinct cavities are the auricles ; the ventricles 

 communicate so freely with one another, that they are to be considered 

 as only one cavity ; therefore these [animals with the above structure 

 of heart] may be called Tricoilia. There the blood from the lungs, 

 and that which has gone through the other parts of the body, mix 

 together, instead of being separated, as in the more perfect animals ; so 

 that some of the last sort is thrown back through the body again with- 

 out passing previously through the lungs, and some of the first sort is 

 pushed a second and perhaps a third time through the lungs, without 

 being first employed in the general circulation. 



The veins of each auricle enter distinctly, as in the first division of 

 hearts ; but the arteries arising from this mixture of ventricles are 

 more complicated. They are not exactly the same as to anatomical 

 structure in all of this class, although much the same as to use, pro- 

 ducing nearly the same effect in all of them. To give an idea of this, 

 we shall describe them in the turtle (Chelone Mydas), which will 

 sufficiently explain their use in all the others \ 



From the compound ventricle of the heart in the Tricoilia arise three 

 arteries, two of which are anterior, the third posterior. Of the anterior, 

 that to the left hand, which is also the largest, is the pulmonary artery, 

 going to the lungs nearly, as in Man ; that to the right hand is the left 

 aorta : and the single posterior artery is the right aorta, which alone 

 gives off the carotids and subclavians. These two aortas make a curva- 

 ture downwards, descend together along the back, and when got to 

 about the middle of the cavity of the animal, unite into one trunk. This 

 union is similar in some degree to the union of the two arteries coming 

 from the gills in fish. 



This description is taken from the turtle, and although it may not 

 exactly agree with all of the above class, as the frog, &c, yet it will in 

 the essentials. Here the lungs and the whole body are evidently 

 supplied from portions of the same mass of blood. But no use appears 

 for the two aortas in this division, and they seem so very superfluous, 

 that one might be tempted to suspect they were only provided to lay 

 the foundation of an analogy with the animals of the immediately in- 

 ferior class, the Pneumobranchiata-,and of the class below them, the Fish 3 . 

 For in fish that have only one auricle and one ventricle, the single 



1 [Hunt. Preps. Nos. 918, 919, 920.] 2 [lb. Wos. 912-917.] 



3 [lb. Xos. 904-911.] 



