KEPTILIA. 



259 



proper, but in some cases they are carried by other bones of the 

 mouth. In the Tortoises and Turtles there are no teeth, and the 

 jaws are simply sheathed in horn, so as to constitute a kind of beak, 

 like that of a bird. The integumentary skeleton is in the form of 

 scales, sometimes combined with bony plates. In the Tortoises and 

 Turtles the integumentary skeleton 

 is so united with the true skeleton 

 as to form a kind of bony case or 

 box, in which the body is enclosed. 



The digestive system presents 

 little worthy of special notice, ex- 

 cept that the termination of the 

 intestine {rectum) opens into a 

 cavity called the " cloaca," which 

 receives also the ducts of the urin- 

 ary and generative organs. 



It is, however, in the structure 

 of the circulatoiy and respiratory 

 organs that the most important 

 characters of the Reptiles are to 

 be looked for. The heart in all 

 Reptiles may be regarded as being, 

 in function, three - chambered, be- 

 ing composed of two auricles and 

 a. single ventricle, imperfectly di- 

 vided by an incomplete partition. 

 In the Crocodiles alone the heart 

 is, struct\i/rally, four - chambered, 

 the ventricle being divided into 

 two by a complete pai'tition. 

 Here, however, the same results 

 are brought about as in the other 

 Reptiles, by means of tx. communi- 

 cation which svibsists between the 

 great vessels which spring from 

 the ventricles thus formed. In the 

 ordinary Reptiles the course of the 

 circulation is, roughly speaking, as 



follows (fig. 186) : The impure or venous blood that has circulated 

 through the body is poured by the gi-eat veins into the right auricle 

 (a). The pure or arterial blood that has been submitted to the 

 action of the lungs is poured by the pulmonary veins into the left 

 auricle (a'). Both auricles empty their contents into the ventricle, 

 and as the partition which divides the ventricle is an incomplete 



ig. 186. — Sketch-diagram of the circu- 

 lation of a Reptile, a Right auricle, 

 receiving venous blooil from the 

 body ; a' Left auricle, receiving arte- 

 rial "blood from the lungs ; v Arterio- 

 venous ventricle, containing mixed 

 blood, which is driven by (p) the pul- 

 monary artery to the lungs, and by (o) 

 the aorta to the body. (The venous 

 system is left light, the arterial sys- 

 tem is blacl<, and the vessels contain- 

 ing mixed blood are cross-shaded.) 



