446 



MOTION. 



require some organs modified to render them 

 capable of reptation. For this purpose we 

 find the spinal column is composed of a greater 

 number of vertebra than is to be found in any 

 other class of animals. The number of verte- 

 brae varies in different species of Ophidia. In 

 a large Python three vertebrae in their normal 

 positions measured one inch, which in an ani- 

 mal of six feet in length would give 216 verte- 

 brae of equal dimensions ; towards the tail, how- 

 ever, their bulk diminishes, by which the flexi- 

 bility of that part is augmented ; the vertebrae 

 are most numerous in some of the smaller spe- 

 cies of Ophidian Reptiles. The bodies of the 

 vertebra are short, and are articulated together 

 by a ball and socket joint which is situated at 

 the inferior border of the body of the vertebra; 

 but as this kind of articulation permits of a ro- 

 tatory motion in every direction, it would ren- 

 der the whole spine exceedingly weak if the 

 motion of the vertebrae was not restricted in 

 some other part ; to attain this object, and to 

 give steadiness and precision in their move- 

 ments, the articulating processes are elongated 

 and furnished with double articular surfaces; of 

 these the inferior is horizontal, and the superior 

 oblique. The horizontal articular surface of 

 one vertebra projects backwards as far as the 

 extreme convex head of the ball, whilst that of 

 the next vertebra projects forward as far as the 

 edge of the socket ; by this arrangement the ho- 

 rizontal articular surfaces are in contact to the 



extent of the depth of the socket. The superior 

 oblique articular surfaces of corresponding por- 

 tions of the two vertebra; are also locked into 

 each other when the vertebral column is ex- 

 tended ; all the articulating processes being 

 cuneiform and fitting into cavities or upon cor- 

 responding surfaces effectually prevents any 

 twisting of the body around it. The areas of 

 the planes of the articular surfaces are suffi- 

 ciently extensive to enable the animal to rotate 

 each vertebra laterally 15 without causing 

 them to slide from each other, consequently six 

 successive vertebra allow of a sufficient range 

 of motion to render the animals capable of 

 turning certain portions of its bodyatrightangles 

 to each other. Owing to the position of the ar- 

 throdial joint, a small amount of flexion on the 

 abdominal aspect in the mesial plane is suffi- 

 cient to produce a considerable space between 

 the spinous processes, so that the motion of 

 the spine in this direction is rather restricted, 

 and from the same cause the spine cannot be 

 but very slightly flexed on the dorsal aspect 

 in the mesial plane without dislocation. The 

 movements of the spine for the purposes of 

 locomotion are, therefore, chiefly lateral. The 

 ribs which extend from the atlas to the anus 

 are articulated to the short transverse pro- 

 cesses of the vertebrae, and in consequence of the 

 absence of the sternum, scapula and pelvis, 

 are endowed with great freedom of motion ; 

 they act in pairs on the transverse abdominal 



Fig. 240. 



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The Octopus milgaris represented in the act of creeping on the short- , its back being turned towards the 

 spectator, towards whom it is supposed to be advancing. 



