660 MAMMALIA. 



except the common hare, and forms the blubber of whales 

 and seals. Beneath the skin is a thin sheet of muscle (the 

 panniculus carnosus), and when this is removed with the 

 skin, many of the muscles of head and neck, limbs and 

 trunk are disclosed. [The student who wishes to study 

 these, and to compare them with their homologues in man, 

 will find practical directions in Parker's ZootomyJ\ 



The Skeleton. t 



The bones, like those of other Vertebrates, are developed 

 either as replacements of pre-existent cartilages, or indepen- 

 dent of any such preformations, but in all cases through 

 the agency of active periosteal membranes. 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 exo- 

 skeleton in any mammals except the armadillos, unless we 

 rank the teeth, which develop in connection with the skin 

 of the jaws, as in a sense exoskeletal. The vertebral centra 

 of Mammals, except in Monotremes and Sirenians, have 

 distinct terminal epiphyses, and the same distinctness of 

 ossification is seen in many of the larger bones. 



Vertebral Column. 



The vertebrae may be grouped as cervical (seven in 

 number), thoracic (with 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 inter-vertebral discs of fibro-cartilage. 



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. 



