MOTION. 



449 



land and water. The outline of the Sauria 

 presents a lengthened form, having the atlanto- 

 sacral axis much greater than the transverse ; 

 they partake more of the figure of the Ophidian 

 than of the Chelonian reptiles, but differ from 

 the former in having legs for the support of 

 the trunk and for locomotion, and also in having 

 a more complex skeleton, including besides the 

 vertebral and costal bones, scapulae, clavicles,* 

 sternoid, pelvic, and sacral elements, as in 

 mammiferous quadrupeds. The acetabula of the 

 scapulae and ossa ilii are inclined horizontally 

 outwards; the humerus and femur, which are 

 short, take the same direction; the ginglymoid 

 articulations of the elbow and knee joints are 

 inclined backwards eccentrically to those of the 

 shoulder and thigh. The effect of this is that 

 the extension and flexion of the fore-arm and 

 leg, being made in the plane of the trans- 

 verse horizontal section of the body, are at the 

 same time movements of abduction and ad- 

 duction ; an arrangement which renders the ex- 

 tremities ill adapted for rapid progression on 

 land. Those Sauria which have the four legs 

 nearly of an equal length may be considered 

 '.he best adapted for locomotion ; the vertebral 

 column being in this case parallel to the plane 

 of motion. When the legs are nearly of equal 

 length, the bone? of the an tenor and posterior ex- 

 tremities bear the following proportions : in the 

 arm of the Crocodilus acutus the liumerus is 

 found to be 4.334, the ulna 3 083; and in the 

 leg, the femur 4.666, the tibia 3.5 inches, so 

 that 4.334 + 3.083 4.666 3.5 = 0.75 

 inch for the difference of the length of these 

 bones. The metatarsus and toes are longer 

 and broader than the carpus and phalanges of 

 the fingers, and present a large surface to strike 

 the water in swimming.f The posterior extre- 

 mities of the Biporcatus are palmated,} those 

 of the Caymati semipalmated. The legs of 

 the Sauria are very short compared with the 

 length of the animal, (which, in the Ripor- 

 -catus, is more than 10 feet,) and their hori- 

 zontal inclination tends still more to depress 

 the centre of gravity towards the plane of mo- 

 tion. From the same cause the legs act on 

 land at a great mechanical disadvantage. 



In the Crocodilean Sauria, the cervical ver- 

 tebrae have but a limited lateral motion owing 

 to the projection and interposition of the false 

 fibs; the dorsal vertebrae have their transverse 

 processes elongated and fixed to the ribs, which 

 have no tubercles, consequently there is but 

 little lateral motion of the back; the lengthened 

 tail, however, admits of considerable lateral 

 play and is of great use in swimming. The 

 Crocodile cannot curve its trunk abruptly or 

 turn it at an acute angle with facility; it runs, 

 however, with considerable agility in a right 

 line. The bones of the skeleton are of a 

 fibrous spongy character, which diminishes the 

 specific gravity of the animal, and is a great 



* The Crocodile has no clavicle. 



t Hunterian Museum. 



f The same remarks ap;ily generally to the Alli- 

 gators of America as to the Crocodiles of the Old 

 World. 



VOL. III. 



advantage in its swiftest mode of progression, 

 which is in a fluid medium. 



Laccrta. The Lacertine Saurians are smaller 

 in dimensions than the Crocodilean, and pos- 

 sess much greater mobility of the vertebral 

 column; the prolongation of tlve animal in the 

 axis of its mesial section is much greater than 

 in its transverse section. As in the C'toco- 

 dilean species, most of the Lizards are pro- 

 vided with four legs, but a very few species 

 have two only. In a few forms, such as the 

 Chirotes and Bipes, the animal cannot support 

 the body above the surface of motion, and 

 consequently drags the thoracic and abdominal 

 segments along upon the ground. In con- 

 sequence of the construction of the hands, 

 claws, and prehensile tail, many Lizards climb 

 with facility. The Gecko is provided with a 

 pneumatic apparatus which it employs in a 

 manner similar to that of the house-fly ; the 

 under surface of each of the five toes, which 

 (with the exception of the thumb) terminate 

 in a sharp claw, is furnished both in the fore 

 and hind feet with as many as sixteen trans- 

 verse plica?, which open into as many cavities 

 or sacs (fig. 244). The contraction of the 

 muscles, acting upon these plicae and sacs, 

 erects the former and dilates the cavities of the 

 latter ; the serrated edges being at the same 

 time accurately applied to any smooth surface, 

 a vacuum is produced, and by this structure 

 the animal is enabled to climb up the vertical 

 planes of walls, and to walk in an inverted 

 position on the ceilings of rooms. The Anolis 

 and Tupenambis, as well as the Gecko, run 

 with considerable speed, and have the power 

 of leaping a great distance; others propel them- 

 selves either backwards or forwards by applying 

 two or more parts to the 

 ground as a fulcrum, and 

 by the alternate flexion 

 and extension of the body, 

 aided by the long and 

 flexible tail. 



In the Thecadactytus 

 the toes are expanded and 

 furnished at their lower ex- 

 tremities with transverse 

 folds ; these folds are di- 

 vided by a deep longitu- 

 dinal groove, in which the 

 claw can be entirely con- 

 cealed ; by this provision 

 the claws are preserved 

 sharp for climbing. 



In the Ptj/odac/ylux, 

 the toes are flattened into 

 plates, the lower parts of 

 which are striated like a 

 fan ; the middle of the fold is cleft, and the 

 claw is fixed in the fissure. All the toes have 

 the claws very much curved. This peculiar 

 organization of the feet enables the animal to 

 climb with great facility. 



In order to allow of the more secure pre- 

 hension of its insect food on the agitated 

 branches of trees, the Cameleon has short, 

 strong, muscular limbs ; a strong, flexible, and 

 prehensile tail ; two thumbs opposite to three 



2 c 



Fig. 244. 



