48 ON THE PLACE OF MAN IN THE SCALE OF BEING. 



cavities, one of which receives the blood from the lungs, and another from the 

 general system; the arterial and the venous blood contained in these two 

 auricles respectively, are transmitted to the third or propelling cavity, the 

 ventricle, where they are mixed; and the half-arterialized fluid is then trans- 

 mitted to the system at large, a part being sent to the lungs. Thus only a 

 portion of the blood expelled from the heart is exposed to the influence of the 

 air ; and that which is transmitted to the body is very imperfectly arterialized. 

 In some of the higher Reptiles, as the Crocodile, the ventricle is double, as 

 in the superior Vertebrata ; and the course of the circulation is so arranged, 

 that pure arterial blood shall go to the head, where it is most required, 

 whilst a mixed fluid is sent to the rest of the body. This plan exactly cor- 

 responds with the one which is adopted in the circulation of the Human 

 foetus, from the time of the formation of the four cavities in its heart, and of 

 the permanent system of vessels, up to the period of birth. The imperfect 

 arterialization of the blood in Reptiles causes a great degree of general inert- 

 ness in their functions. Their motions are principally confined to crawling and 

 swimming ; their general habits are sluggish, and their sensations are obtuse ; 

 and their nutritive functions are very slowly performed. Hence they can 

 exist for a long time with a very feeble exercise of these functions, under cir- 

 cumstances that would be fatal to animals in which they are performed with 

 greater activity. In cold and temperate climates, they pass.the whole winter 

 in a state of torpidity; and at other seasons, they may be kept during a long 

 time from their due supplies of food and air, without appearing to suffer much 

 inconvenience. 



40. In regard to the structure of their skeleton, and the external form of the 

 body, there is a considerable difference among the sev'eral orders of Reptiles. 

 Thus, Tortoises, Lizards and Serpents, differ from each other so widely, that a 

 common observer would separate them completely ; and yet they not only agree 

 in all the foregoing characters, but pass into one another by links of transition so 

 gradual, that it is even difficult to classify them. They differ, however, more 

 in the configuration of the accessory parts than in the structure of the essential 

 portion of the skeleton, the spinal column. This is characterized by the 

 ball-and-socket articulation of the vertebrae, each vertebra liaving one surface 

 convex, and the other concave, a structure which is more strongly marked in 

 Serpents, whose movements are performed chiefly by the flexion of the spinal 

 column itself, than it is in the other tribes. The chief characteristic of the 

 Tortoise tribe, is the shell or case in which the body is contained. The upper 

 arch of this shell, termed the carapace, is formed by a bony expansion from 

 the edges of the ribs, which is covered by a set of horny plates, that are to be 

 regarded (like smaller scales) as epidermic appendages. The under portion, 

 termed the plastron, is composed of the sternum, which is in like manner 

 extended laterally. In the land-tortoises, this usually forms a complete floor; 

 but in the aquatic species, a part is commonly absent, the interval being filled 

 up by cartilage and membrane. The skeleton of the Lizards is formed more 

 upon the general plan of that of Mammalia, but may be readily distinguished 

 from it. The sternum is usually prolonged over the front of the abdomen, 

 and the ribs are continued through a much larger part of the spinal column ; 

 of these abdominal ribs, the white lines across the recti muscles in the higher 

 Vertebrata are evidently the rudiments. In the higher Lizards, the power of 

 locomotion is almost entirely delegated to the extremities ; but in the less typi- 

 cal species, the body and tail are much prolonged, so as to present a serpenti- 

 form aspect ; and first one pair of feet, and then the other disappear, until the 

 form is altogether that of the Serpent. Even in Serpents, however, rudiments 

 of extremities are frequently to. be found; but their mode of progression is very 

 different, and these rudiments are of no assistance to them. The most re- 



