SKELETON. 675 



By themselves, however, must be ranked little sesamoid 

 bones, which are developed within tendons and near 

 joints, notably, for instance, the patella or knee-pan. 

 There is no bony exoskeleton in any mammals except the 

 armadillos, unless we rank the teeth, which develop in con- 

 nection with the skin of the jaws, as in a sense exoskeletal. 



The vertebrae may be grouped in five sets : cervical 

 (seven in number), thoracic (with well-developed ribs), 

 lumbar (without ribs), sacral (fused to support the pelvis), 

 and caudal. The faces of the centra are more or less flat, 

 and between adjacent vertebrae there are intervertebral 

 discs of fibro-cartilage. A vestige of the notochord is found 

 in Mammals in the gelatinous nucleus pulposus in the centre 

 of the intervertebral discs. 



The first vertebra or atlas is ring-like, its neural canal 

 being very large, its centrum unrepresented except by the 

 odontoid process, which fuses to the second vertebra. The 

 ring is divided transversely by a ligament, through the upper 

 part the spinal cord passes, into the lower the odontoid 

 process projects. The transverse processes are very broad ; 

 the articular surfaces for the two condyles of the skull are 

 large and deep. 



The second vertebra or axis has a broad flat centrum pro- 

 duced in front in the odontoid process. The neural spine 

 forms a prominent crest, the transverse processes are small, 

 the anterior articular surfaces are large. 



A typical lumbar vertebra will show the centrum and its 

 epiphyses, the neural arch and neural spine, the transverse 

 processes, the anterior and posterior articular processes or 

 zygapophyses, the median ventral hypapophysis. the small 

 anapophyses from the neural arch below the posterior 

 zygapophyses, below the anapophyses the posterior inter- 

 vertebral notches passages through which the spinal nerves 

 pass out, and anteriorly a similar pair of notches. There are 

 twelve or thirteen pairs of ribs which support the wall of the 

 thorax, and aid in the mechanism of respiration. The first 

 seven pairs articulate with the breast-bone, the eighth and 

 ninth are connected to the ribs in front, the others are free. 

 Any one of the first seven or more typical ribs consists of 

 two parts, a vertebral portion articulating with a vertebra, an 

 imperfectly ossified sternal portion connecting the end of 



