THE COCCYGEAL VERTEBH^ 43 



laris), which articulates with the ihum; it is slightly concave in its length, and 

 somewhat rough and irregular. The rest of the dorsal surface of the wing is rough- 

 ened for ligamentous attachment, while the ventral surface is smooth. 



The apex (Apex ossis sacri) is the posterior aspect of the last sacral vertebra 

 and IS quite small. , It presents the elliptical flattened surface of the body above 

 which IS the triangular posterior opening of the sacral canal, surmounted by the 

 last sacral spine. There is a pair of narrow notches between the arch and body 

 above which rudiments of articular processes may occur. ' 



The name sacral canal (Canahs sacralis) is applied to that part of the vertebral 

 canal which traverses the sacrum. Its anterior part is large and has the form of 

 a triangle with the angles rounded off; its width is nearly twice its height. Traced 

 backward it is seen to diminish in size rapidly, and the posterior opening is small 

 and triangular. 



The term lateral part (Pars lateralis) designates the portion lateral to the 

 foramina, which results from the fusion of the transverse processes. 



Development.— The several sacral vertebrsB ossify in the typical manner, 

 feeparate centers for costal elements in the lateral parts have not yet been found in 

 the domesticated animals. Fusion begins in front, and is usually not complete 

 till adult age.i The lateral parts unite before the bodies. It is rather curious 

 that the epiphyseal plates of adjacent segments unite with each other before they 

 fuse with the main portion of the bodies. 



THE COCCYGEAL VERTEBRA 



The coccygeal vertebra (Vertebraj coccygeal) vary considerably in number, but 

 eighteen may be taken as an average. From first to last they become reduced 

 in size and, with the exception of a few at the beginning of the series, consist of 



Spinous process Spinous process 



Vertebral foramen Vertebral foramen 



, , Arch 



A rch 



Transverse process 



, Transverse proc- 



ess 



Groove 

 Body Boriy 



Fig. 20. — First CoccygEAL Vertebra of Horse; Fig. 21. — Second Coccygeal Vertebra of Horse; 



Posterior View. Posterior View. 



Leader to arch points to rudimentary articular process. 



bodies only. The first three have bodies which are somewhat flattened dorso- 

 ventrally, constricted in the middle, and have at the ends convex, elliptical, artic- 

 ular surfaces. The ventral surface has a median groove (Sulcus vasculosus) for 

 the coccygeal artery. The arch is small and triangular; it is formed of two flat 

 plates which are prolonged to form a short spinous process with a thickened and 

 often double summit. The anterior notches are absent. Functional articular 

 processes are not present, but small rudiments of the anterior pair commonly occur. 

 The transverse processes are relatively large plates which project horizontally 

 outward. Further back the arch becomes incomplete dorsally, and soon disap- 

 pears; the transverse processes gradually fade out, and the vertebrae are reduced 

 to cylindrical rods of diminishing size. The last one has a pointed end. 



' It is not rare to find fusion of some of the bodies incomplete even in adult subjects. 



