700 MAMMALIA. 
hare; it forms the blubber of whales. Beneath the skin 
is a thin sheet of muscle (the panniculus carnosus), by 
means of which the skin can be twitched, as in horses, 
etc., and when this is removed with the skin, many of the 
muscles of head and neck, limbs and trunk, are disclosed 
(see Parker’s Zootomy). 
Skeleton.—The bones, like those of other Vertebrates, 
are developed either as replacements of pre-existent cartil- 
ages, or independent 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 exoskeleton 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 vertebree 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 vertebrze 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 
tring is divided transversely by a ligament, through the 
upper part the spinal cord passes, into the lower the odon- 
toid 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 
produced 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 ort 
zygapophyses, the median ventral hypapophysis, the small 
anapophyses from the neural arch below the posterior 
