xin PHYLUM CHORD ATA 479' 



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 vertebrae in the cervical region ; 

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

 Sloth, and the three-toed Sloth. The number of thoracic and 

 lumbar vertebrae is not so constant; usually there are between 

 nineteen and twenty-three. Hyrax has a larger number of 

 thoracico-lumbar vertebras than any other Mammal from twenty- 

 nine to thirty-one. 



The thoracic vertebrae have ribs which are connected, either 

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

 them with the sternum. Each rib in general 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 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 vertebras, 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 Mammals there are developed a series of dicn'on bones- 

 V-shaped bones, which are situated opposite the intervertebral 

 spaces. 



The centrum of each vertebra ossifies from three centres 1 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 less distinct in the Monotremes, and in the 

 Dugong (Sirenia) have not been detected. Between successive 

 centra are formed a series of discs of fibro-cartilage the inter- 

 vertebral discs - - represented in lower Vertebrates only in 

 Crocodiles and Birds. The anterior and posterior surfaces of the 

 centra are nearly always flat. 



The sternum consists of a number of segments the pre-sternum 

 in front, the meso-sternum or corpus stcrni, composed of a number* 

 of segments or sterncbrce, in the middle, and the xipJii-stcnmm 

 behind. The sternum is in great part, though not completely.. 



1 Usually the two centres of ossification which form the neural arches also> 

 contribute to the formation of the bony centrum. 



