VEBTEBB^. 25 



from one end to the other, transforms it into a ring in which we recognize, for 

 facility of description, two parts — the one inferior, the other superior. The first, 

 or hotly, is very thick, and forms the base of the vertebras ; the second, which 

 is thin, has been designated spinous or spinal — from one of the pecuUarities it 

 presents, or annular — because it circumscribes the major portion of the spinal 

 foramen. This division is not altogether an arbitrary one, for the body and the 

 annular portion constitute, in the foetus, two distinct pieces, which do not become 

 united for a long time after birth. 



Body {centrum). — The shape of the body of a vertebra is that of a prism 



ELEMENTS OF A VERTEBRA. (AFTER OWEN.) 



A, Ideal typical vertebra ; B, Actual thoracic vertebra of a bird ; c, Centrum (or 

 body), giving off, d, d, the diapophyses, and p,p, the parapophyses (transverse and 

 articular processes) ; the neural arch, enclosing the spinal cord, is formed by 

 n, n, the neurapophyses (lamince), saxd n, s, the neural spine {spinous process); 

 the hamal arch, enclosing the great centres of the circulation, is formed by h, h, 

 the haemapophyses (costal cartilages) ; and h, s, the haemal spine (sternum). 

 From both the neurapophyses and haemapophyses may be given off the zygapo- 

 physes, z, z. The lateral arches, which may enclose the vei'tebral arteries, o, o, 

 are completed by the pleurapophyses (ribs), pi. ; these in B are bent downwai-ds, 

 so as to form part of the hasmal arch, and give off the diverging appendages, a, a. 



with four faces, of which two only — the superior and inferior — are free, and can 

 be studied in the adult ; the two lateral faces are united and confounded with 

 the annular portion. This prism also presents two extremities — an anterior and 

 posterior. 



Faces. — The superior face, limited in extent, forms part of the spinal foramen, 

 constituting its floor. It exhibits : 1. On the middle line, two roughened, 

 prominent surfaces, representing two triangles, whose summits are opposed. 

 2. On the sides, two depressed smooth surfaces, perforated by one or more 

 openings that lead to the interior of the bone. The inferior face is divided into 

 two lateral portions by a median crest (the htemal spine). 



Extremities. — The anterior has a prominent convex head, more or less 

 detached. The posterior offers a cavity for the reception of the head of the 

 next vertebra. These two planes — the one convex, the other concave — do not 

 come into immediate contact ; an elastic, flexible fibro-cartila^e, firmly attached 

 to each, is interposed between them. 



