INTRODUCTION. 



33 



shaped depressions or sockets, the intervertebral substance, 

 the prominence or ball. 



The same may be said of the general arrangement of the 

 muscles in the trunk and tail of the Cetacea, the principal 

 muscles in this case being distributed, not on the sides, but 

 on the dorsal and ventral aspects. The lashing of the tail 

 in the whales is consequently from above downwards or 

 vertically, instead of from side to side. The spinal column is 

 jointed as in the fish, with this difference, that the vertebrae 

 (especially towards the tail) form the rounded prominences or 

 ball, the meniscus or cup-shaped intervertebral plates the 

 receptacles or socket. 



When limbs are present, the spine may be regarded as 

 being ideally divided, the spiral movements, under these 

 circumstances, being thrown upon the extremities by typical 

 ball-and-socket joints occurring at the shoulders and pelvis. 

 This is peculiarly the case in the seal, where the spirally 

 sinuous movements of the spine are transferred directly to 

 the posterior extremities.-*- 



The extremities, when present, are provided with their 

 own muscular cycles of extensor and flexor, abductor and 

 adductor, pronator and supinator muscles, — these running 

 longitudinally and at various degrees of obliquity, and en- 

 veloping the hard parts according to their direction — the 

 bones being twisted upon themselves and furnished with 

 articular surfaces which reflect the movements of the 

 muscular cycles, whether these occur in straight lines an- 

 teriorly, posteriorly, or laterally, or in oblique lines in inter- 

 mediate situations. The straight and oblique muscles are 

 principally brought into play in the movements of the extremi- 



^ That the movements of the extremities primarily emanate from the spine is 

 rendered probable by the remarkable powers possessed by serpents. " It is 

 true," writes Professor Owen {op. cit. p. 261), that the serpent has no limbs, 

 yet it can outclimb the monkey, ontswim the fish, outleap the jerboa, and, 

 suddenly loosing the close coils of its crouching spiral, it can spring into the 

 air and seize the bird upon the wing." .... "The serpent has neither 

 hands nor talons, yet it can outwrestle the athlete, and crush the tiger in the 

 embrace of its ponderous overlapping folds." The peculiar endowments, 

 which accompany the possession of extremities, it appears to me, present 

 themselves in an undeveloped or latent form in the trunk of the reptile. 

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