754 MAMMALIA. 



the fishes even in their own element ; and the gigantic Whales, wallow- 

 ing upon the surface of the sea, " tempest the ocean" in their fury. 



(2146.) With habits so diverse, we may well expect corresponding 

 diversity in their forms, or in the structure of their limbs ; and, in fact, 

 did we not compress our description of these particulars into reasonable 

 limits, we might easily test the perseverance of the most patient reader 

 in following us through the mass of details connected with this part of 

 our subject. We shall therefore, commencing as we have hitherto 

 done, with the osteology of the class, first describe, in general terms, 

 the characters of a Mammiferous skeleton, and then, as we arrange the 

 Mammalia under the various orders into which they have been distri- 

 buted, speak of the most important aberrations from the given type. 



(2147.) The vertebral column of all Mammals, with the remarkable 

 exception of the Cetacea, is divisible into the same regions as in the 

 human skeleton, viz. the cervical, dorsal, lumbar, sacral, and coccygeal 

 or caudal portions. 



(2148.) The cervical vertebra are invariably seven in number. The 

 Sloth (Bradypus tridactylus) was, until recently, regarded as forming 

 a solitary exception, it having been supposed to possess nine cervical 

 vertebra ; the researches of Professor Bell, however, show that even 

 this animal conforms to the general law. The distinguished naturalist 

 referred to has demonstrated *, " that the posterior two of these verte- 

 bra3 have attached to them the rudiments of two pairs of ribs, in the 

 form of small elongated bones articulated to their transverse processes ; 

 they must therefore be considered as truly dorsal vetebras, modified 

 into a cervical form and function suited to the peculiar wants of the 

 animal." Professor Bell further observes that " the object of the in- 

 creased number of vertebrae in the neck of the Sloth is evidently to allow 

 of a more extensive rotation of the head ; for, as each of the bones turns 

 to a small extent upon the succeeding one, it is clear that the degree of 

 rotation of the extreme point will be in proportion to the number of 

 pieces in the whole series. When the habits of this extraordinary 

 animal are considered, hanging as it does from the under surface of 

 boughs, with the back downwards, it is obvious that the only means by 

 which it could look towards the ground must be by rotation of the 

 neck ; and as it was necessary, to effect this without diminishing the 

 firmness of the cervical portion of the vertebral column, to add certain 

 moveable points to the number possessed by the rest of the class, the 

 additional motion was acquired by modifying the two superior dorsal 

 vertebra), and giving them the office of cervical, rather than by infring- 

 ing on a rule which is thus preserved entire without a single known 

 exception." 



(2149.) The occipital bone articulates with the atlas by two lateral 

 condyles, instead of by a single central articulating surface, a circum- 

 * Cyclop, of Anat. and Phys. art. EDENTATA. 



