CHAP, in.] SKELETON OF THE BEAD AND TRUNK. 



45 



Its first component vertebra is considerably larger than the two suc- 

 ceeding ones, which are about equal in size. 



The ventral surface of the sacrum (A) is markedly concave from 

 before backwards, and is also concave transversely at its more an- 

 terior ^part. It is marked by two transverse lines (which indicate 

 the original limits of the vertebral centra), and at each end of each 

 line is a considerable aperture or foramen (/). These four open- 

 ings, called the ventral sacral foramina, give exit to the anterior 

 divisions of the sacral nerves. 



Fig. 22. THE SACRUM. 



A. Ventral view. 



B. Dorsal view. 



C. Front view. 



c. Anterior central articular surface. 

 /. Foramina. 



I. Lateral masses. 



m. Rudiments of zygapopliyses and metapo- 



physes conjoined. 

 s. Neural spines. 

 z. and 3. Zygapophyses. 



The dorsal surface of the sacrum is rough, and exhibits three 

 neural spines (s) projecting nearly straight upwards. They are all 

 shorter than the neural spine of the last lumbar vertebra, and the 

 third sacral neural spine is much smaller than the two in front of 

 it, whereof the first is the taller. 



External to the neural spines, and at the outer margins of the neural 

 lamina) (which form a completely roofed neural canal throughout 

 the sacrum), there are, on each side, four eminences, representing 

 zygapophyses or metapophyses, or both(w). Thus at the anterior 

 end of the centrum we have (s) on each side a prezygaponhysis 

 (with its outer margin prolonged by the metapophysis), which is 

 like that of the last sacral vertebra, except that it is somewhat 

 larger. Behind this there is a smaller prominence, which represents 

 the conjoined metapophysis and zygapophysis anchylosed together 

 at the junction of the first and second sacral vertebra. Behind this, 

 again, there is another still smaller prominence which represents the 

 same parts at the junction of the second and third sacral vertebra?. 

 Behind this again, and close behind the third sacral neural spine, is 

 a third process (which is the postzygapophysis of the third sacral 

 vertebra) which articulates with the first vertebra of the tail (5). 



Just external to each process formed of coalesced and anchylosed 

 zygapophyses, is a considerable aperture or foramen. There are 

 four such, and these are termed the dorsal sacral foramina, and they 



