VERTEBRAL COLUMN 



49 



reaiain distinct. That the latter is the more original condition in 

 Birds is shown by a study of their development as well as by the 

 condition of the tail in Archseopteryx, in which it was supported by 

 numerous elongated free vertebrae (Fig. 38j. Moreover, in many 

 Birds {e.g. Psittacus undulatus) more vertebrae are formed in the 

 embryo than are seen in the adult. It must, however, be borne 

 in mind that the pygostyle may be made up of from six to ten 

 fused caudal vertebrae, and in the sacrum 

 even a greater number may be included 

 (cp. p. 48) : thus in the common Duck, 

 seven become united with the pelvis, 

 eight remain free, and the pygostyle is 

 composed of ten separately ossified and 

 fused segments, making in all twenty- 

 five vertebrae originally present in the 

 caudal region of this Bird. 



Mammalia. — The notochord here 

 persists longer intervertebrally than in- 

 travertebrally, but it disappears entirely 

 by the time the adult condition is reached. 

 A jelly-like pulpy mass, the nucleus 

 2ndposus, persists, however, throughout 

 hfe in the centre of the fibro-cartilaginous 

 menisci which are developed between 

 the centra. The whole vertebral column 

 is preformed in cartilage, and the centra 

 develop in continuity with the arches 

 but become ossified from separate centres, 

 as do also the various processes. These 

 ossifications, however, become fused to- 

 gether in the adult. The presence of 

 bony discs or epiphyses on the ends of 

 the centra which unite with the latter 

 comparatively late, is very characteristic 

 of Mammals ; they are however absent 

 or only imperfectly developed in Mouo- 

 trematas and in existing Sirenia. 



True articulations between the centra 

 are never formed, except on the atlas 



and anterior face of the axis; but as in Amphibians, Reptiles, 

 and Bii-ds, well-developed articular processes (zygapophyses) 

 are present, arising from the neural arches.^ The cervical 

 region is usually the most moveable, and the centra may be so 

 much hollowed out in this region as to give them an opisthoccelous 

 character {e.g. Ungulata). In some cases, on the other hand, the 



^ In certain Edentata {e.g. Myrmecophaga, Dasypus) extra articular processes 

 are present besides the ordinary zygapophyses on the posterior thoracic and 

 lumbar vertebrse (Fig. 39b.). 



E 



Fig. 37. — Pelvis of Owd 

 (Strix bubo). Ventral 

 view. 



W, position of the primary 

 sacral vertebrae : be- 

 tween B and II, and 

 behind W, are seen the 

 secondary sacral verte- 

 brte, fused with the 

 primary (IT) ; //, ilium ; 

 Is, ischium ; P, pubis ; 

 t, foramen between 

 ilium and pubis ; H, 

 last two pairs of ribs. 



