184 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



while the rest forms the neural arch. The successive centra are 

 united firmly together, and are procxlous i.e., concave in front 

 and convex behind. Both surfaces are covered by a thin layer 

 of cartilage. (The 8th vertebra is amphiccelous, concave on both 

 faces.) Dorsal and lateral spaces are left between adjacent arches. 

 The former are filled up by connective tissue, while through the 

 latter, or intervertebral foramina, spinal nerves take exit. From 

 the upper side of each arch a small neural spine projects upwards 

 and backwards in the middle line, while on each side of the arch 

 a stout, cartilage-tipped bar, the transverse process, runs outwards. 

 Four small projections with smooth articular surfaces project 

 from the front and back of the arch. By these articular processes 

 or zygapophyses the adjoining arches are linked together. Two, 

 the pm-zygapophyses, are anterior, and their articular surfaces face 

 upwards and inwards, while the other two, post-zygapophyses, are 

 posterior, and their articular surfaces face downwards and out- 

 wards, overlapping the prse-zygapophyses of the following vertebra. 



The 1st vertebra, or atlas, is devoid of tran verse processes and 

 prse-zygapophyses, and the neural spine is rudimentary. The 

 centrum is somewhat thin, and projects forwards between the 

 occipital condyles, which articulate with two large concave facets 

 on the front of the vertebra. 



The 9th vertebra, the sacrum, has very large and strong trans- 

 verse processes directed outwards and backwards. The centrum 

 is convex in front, and presents a pair of articular tubercles behind, 

 which articulate with corresponding concavities on the front of 

 the urostyle. This is a bony rod, somewhat trihedral, and taper- 

 ing behind to a point tipped with cartilage. It may be regarded 

 as representing a number of vertebrae fused together, of which a 

 ridge running along its dorsal surface corresponds to the united 

 neural spines. Two small foramina open, one on either side, 

 into the canal of the urostyle, a short distance behind its anterior 

 end. Through these the coccygeal nerves take exit, and they, 

 therefore, correspond to intervertebral foramina. The part of the 

 urostyle in front of them is the first of the fused vertebrae, and 

 may possess more or less distinct transverse processes. 



(c) The sternum consists of several cartilage bones and cartilages 

 placed in the middle line on the ventral side of the thoracic region. 

 In the extreme front a flat, narrow bone, the omo-sternum, is found, 

 to the anterior end of which a rounded flap of cartilage (the epi- 



