22 Mammalia. 



a character of many extinct reptiles : but, as distinctive of 

 Mammalia, note that these articular surfaces are in this class 

 developed as separate, discoidal, epiphysial plates, which continue 

 for a longer or shorter period of life, and then coalesce with 

 the body. Each vertebra is developed from five centres .of 

 ossification : two for the epiphyses, two for the neural arch, 

 and one for the body : but the sutures between the elements of the 

 neural arch and the centrum are early ohliieratedy generally be- 

 fore the epiphyses coalesce with the centrum. The centra are 

 articulated by concentric ligaments, composed of fibrous tissue 

 and cartilage, forming the so-called " intervertebral cartilages." 

 The vertebral column is generally (? xi.) divided into the same 

 regions as in Man, viz., cervical, dorsal, lumbar, sacral, and 

 ca*dal. It exhibits great constancy in the number of verte- 

 brae in the Cervical region : there are generally seven (cf. xii, 

 xiii.) : the great variations in the length of the neck being 

 solely dependent upon that of the centrum of each vertebra 

 (compare centrum of cervical Y. of Giraffe with that of 

 Balasna). The first (atlas) and second (axis) cervical vertebrae 

 present a curious anomaly, in that the centrum of the atlas 

 has in the adult coalesced with that of the axis. The atlas 

 has constantly in Mammalia two articular cavities for the 

 reception of the two condyles of the skull. The cervicals are 

 generally all movable on each other (cf. xi.), and have their 

 transverse processes perforated for the vertebral artery. The 

 number of Dorsal vertebrce varies, but there are seldom fewer 

 than twelve (cf. iv.) or more than fifteen (cf. vii. xiii.) : the spi- 

 nous processes are seldom wanting (cf. iv.), and are generally 

 long and thin, and slope towards one in the latter half of their 

 series which is vertical, rises from the thence named *' anti- 

 clinal" vertebra, and indicates the centre of motion of the trunk. 

 To these spines is attached the ligamentum nuchae, and with 

 it they form what is termed the "withers." The ligamentum 

 nucha? has its anterior attachment on the spine of the second 



