510 ZOOLOGY 



SECT 



fore-limbs acting as crutches is necessary to enable the animal to 

 swinof itself along. 



Endoskeleton. — The spinal column of Mammals varies in" the 

 number of vertebra* which it contains, the differences being mainly- 

 due to differences in the lenoth of the tail. The various recfions 

 are very definitely marked off. In the cervical region the first two 

 vertebras are modified to form the atlas and axis. Owing to the 

 absence of distinct cervical ribs, the posterior cervical vertebrae 

 are much more sharply marked off from the anterior thoracic than 

 is the case in Reptiles and Birds. The vertebrae of the cervical 

 region have double transverse processes (or a transverse process 

 perforated at the base by a foramen) in all except the last. The 

 lower portion of the transverse process in certain cases {e.g., seventh 

 and sometimes some of the others in Man) arises from a separate 

 ossification, and this is regarded as evidence that the lower part, 

 even when not independently ossified, represents a cervical rib. 

 Seven is the prevailing number of vertebrre in the cervical region ; 

 there are only three exceptions to this — the Manatee, Hoffmann's 

 Sloth, and the three-toed Sloth {cf. pp. 525 and 534). The number 

 of thoracic and lumbar vertebrse is not so constant ; usually there 

 are between nineteen and twenty-three. Hyrax has a larger 

 number of thoraco-lumbar vertebrse than any other Mammal — 

 from twenty-nine to thirty-one. 



The thoracic vertebra have ribs which are connected, either 

 directly or by intermediate ribs, with the sternal ribs, and through 

 them with the sternum. Each rib typically articulates with the 

 spinal column by two articulations — one articular surface being 

 borne on the head and the other on the tubercle. The tubercle 

 articulates with the transverse process, and the head usually with 

 an articular surface furnished partly by the vertebra with which 

 the tubercle is connected, and partly by that next in front ; so 

 that the head of the first thoracic rib partly articulates with the 

 centrum of the last cervical vertebra. 



In all the Mammalia in which the hind-limbs exist — that is to say, 

 in all with the exception of the Sirenia and the Cetacea — there is a 

 sacrum consisting of closely united vertebrse, the number of which 

 varies in the different orders. The caudal region varies greatly 

 as regards the degree of its development. In the caudal region of 

 many long-tailed Mammals there are developed a series of chevron 

 hones — V-shaped bones, which are situated opposite the inter- 

 vertebral spaces. 



The centrum of each vertebra ossifies from three centres^ — a 



middle one, an anterior, and a posterior. The middle centre forms 



the centrum proper ; the anterior and posterior form the epiphyses. 



. The epiphyses are almost entirely absent in the Monotremes, 



and in the Dugong {Sirenia) have not been detected. Between 



■ ^ Usually the two centres of ossification wliicli form tlic neural aiches also 

 contril)ute to the formation of tlic bony centrum. 



