XIII 



PHYLUM CHORDATA 



495 



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 vertebrae in the cervical region ; 

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

 Sloth, and the three-toed Sloth (cf. p. 509). The number of 

 thoracic and lumbar vertebras is not so constant ; usually there 

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



FIG. 1136. Gorilla. (From the Cambridge Natural History.) 



number of thoraco-lumbar vertebrae than any other Mammal 

 from twenty-nine to thirty-one. 



The thoracic vertebras 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 



