4o 



GENERAL ANATOMICAL CHARACTERS 



of the trunk, in nearly all mammals which possess completely 

 developed hinder limbs, two or more vertebra? become ankylosed 

 together to form the " sacrum," or portion of the vertebral column 

 to which the pelvic girdle is attached ; (2) in some species of 

 Whales and Armadillos there are constant ossific unions of certain 

 vertebrae of the cervical region. 



Although the vertebrae of different regions of the column of the 

 same animal or of different animals present great diversities of 

 form, yet there is a certain general resemblance among them, or a 

 common plan on which they are constructed, which is more or less 

 modified by alteration of form or proportions, or by the addition or 

 suppression of parts to fit them to fulfil their special purpose in the 

 economy. An ordinary or typical vertebra consists, in the first 

 place, of a solid piece of bone, termed the body or centrum (Fig. 

 9, c), of the form of a disk or short cylinder. The bodies of con- 



az 



JJZ 



Fio. 9. — Anterior surface of Human 

 thoracic vertebra (fourth), c, Body or 

 centrum ; nc, neural canal ; p, pedicle, 

 and I, lamina of the arch ; t, transverse 

 process ; az, anterior zygapophysis. 



Fig. 10. — Side view of the first lum- 

 bar vertebra of a Dog (Canis familiaris). 

 s, Spinous process ; az, anterior zygapo 

 physis ; pz, posterior zygapophysis ; m, 

 metapophysis ; a, anapophysis ; t, trans- 

 verse process. 



tiguous vertebrae are connected together by a very dense, tough, and 

 elastic material called the " intervertebral substance," of peculiar and 

 complex arrangement. This substance forms the main, and in some 

 cases the only, union between the vertebrae. Its elasticity provides 

 for the vertebrae always returning to their normal relation to each 

 other and to the column generally, Avhen they have been disturbed 

 therefrom by muscular action. A process (p) arises on each side 

 from the dorsal surface of the body. These processes, meeting in 

 the middle line above, form an arch, surmounting a space or short 

 canal (nc). Since it contains the posterior prolongation of the 

 great cerebro-spinal nervous axis, or spinal cord, this space is called 

 the neural canal, and the arch the neural arch, in contradistinction 

 to another arch on the ventral surface of the body of the verte- 

 brae, called the haemal arch. The latter is, however, never formed 



