512 ANATOMY OF VERTEBRATES. 



cavity into which it opens, fig. 340, L, is distinct from the 

 pulmonic cavity, the septum being complete : its walls are smooth, 

 or less broken by c columns earners ' than in other Reptiles ; and 

 the free walls of this ventricle are more compactly muscular. The 

 ventricle is produced in a subconical form, from its base to the 

 origin of the right or brachiocephalic aorta : the auriculo-ventri- 

 cular valve is slit, in fig. 340, to show the course of the ventricle 

 to the origin of that aorta : this has a pair of semilunar valves, 

 above which is the intercommunicating orifice with the left 

 aorta. 



Thus the heart, in Crocodilia, consists of two auricles and two 

 ventricles, corresponding to the ( right ' and ' left ' auricles and 

 ventricles of Mammals. But, through the origin of an aorta from 

 the right as well as from the left ventricle, and their intercom- 



o 



munication, it follows that whenever, from an impeded state of the 

 pulmonary circulation, the right ventricle and its arteries become 

 over-distended, the venous blood flows through the inter-aortic 

 orifice into the arterial trunk, which, after supplying the head 

 and fore-limbs, bends, at A' ', over the right bronchus and effects 

 an union at A", fig. 339, with the left aorta, A, h. Such a state of 

 the circulation coincides with and facilitates the long submersion 

 of the Crocodile. When the animal is on land and breathing the 

 air directly, the arterialized blood flows freely into the ventricle, fig. 

 340, L, and the synchronous currents from this and the opposite 

 ventricle throw forward the valves at the respective origins of the 

 two aorta3 and close the interaortal orifice. The arterial and venous 

 streams flow on unmixed ; the former to the brain and other parts 

 of the head and fore-limbs ; the latter, by the branch, h, fig. 339, 

 chiefly to the liver and contiguous viscera ; a small part mixing 

 with the arterial blood in A', to be transmitted by A" to the other 

 abdominal viscera, hind limbs, and tail. To convert the heart of 

 the Crocodile into that of the bird, it needs only to obliterate the 

 left aorta; fig. 339, R, to appropriate the right or pulmonic ven- 

 tricle, exclusively to the service of the pulmonary artery ; and the 

 ( left ' or systemic ventricle to the service of the aorta, which in 

 Hcematotherma is the exclusive distributor of arterial blood, in an 

 unmixed state, to the general system. 



90. Gills of Batracliia. --The blood is conveyed in all Reptiles 

 from the ventricular part of the heart, really or apparently by a 

 single trunk, which, in the one case, is called the ( bulbus arte- 

 riosus,' in the other the 6 conus arteriosus.' The interior develope- 

 ments by which the c bulbus ' is converted into the ' conus,' are 

 interestingly gradational, and the insulation of the pulmonary 



