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



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 perma- 

 nent system of vessels, up to the period of birth. The imperfect arterialisa- 

 tion of the blood in Reptiles, causes a great degree of general inertness 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 

 circumstances that would be fatal to animals, in which they are performed 

 with greater activity. In cold and temperate climes, 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 suf- 

 fer much inconvenience. 



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

 the body, there is a considerable variation among the several 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 struc- 

 ture of the essential portion of the skeleton, the spinal column. This is 

 characterised by the ball-and-socket articulation of the vertebras, each vertebra 

 having one surface convex and the other concave, a structure which is more 

 strongly marked in Serpents, Avhose 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 appen- 

 dages. The under portion, termed the plastron, is composed of the sternum, 

 which is in like manner extended laterally. In the Land-tortoises, this usu- 

 ally 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 typical species, the body and tail are much 

 prolonged, so as to present a serpentiform 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 remarkable feature in the Serpent's 

 skeleton, besides the absence of legs, and the large number of ribs and verte- 

 brae, is the deficiency of a sternum; through the absence of this, the extremi- 

 ties of the ribs are free, and they become in fact the fixed points, on which 

 the animal crawls, when advancing slowly forwards, in a manner which bears 

 a strong resemblance to the progression of the Centipede. 



30. Although the configuration of the cranium varies much in the different 

 orders of Reptiles, yet there is a remarkable agreement in certain general cha- 

 racters, and in the general degree of development. It consists of a much 

 larger number of parts, than are to be found in the cranium of adult Birds or 

 Mammalia ; each principal bone being subdivided, as it were, into smaller ones. 



