THE COCCYGEAL VERTEBR.B 41 



are strong prismatic masses with pointed ends, which result from the fusion of the 

 first with part of the second transverse process. Each has in front a large, oval, 

 slightly convex surface for articulation with the transverse process of the last lum- 

 bar. Posteriorly there is an elongated oval area which faces upward, backward, 

 and outward. This is the auricular surface (Facies auricularis), which articulates 

 with the ilium; it is slightly concave in its length, and somewhat rough and irregu- 

 lar. The rest of the dorsal surface of the wing is roughened for ligamentous attach- 

 ment, 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 (Canalis 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 about twice its height. Traced 

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

 small and triangular. 



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

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



Vertebral canal 



Spine 



-process Spine --\ 



Transverse _ 

 Transverse process 



Body Posterior end of body 



Fig. 20. — First CocrvGEAL Vertebra of Horse, Fig. 21. — First Coccygeal Vertebra of Horse, 

 Left View. (After Schmaltz, Atlas d. Anat. Dorsal View. (After Schmaltz, Atlas d. Anat. 



d. Pferdes.) d. Pferdes.) 



Development. — The several sacral vertebrae ossify in the typical manner. 

 Separate 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 

 behind till near adult age. 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 

 These (Vertebrae coccyges) 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 bodies only. The 

 first three have bodies which are somewhat flattened dorso-ventrally, constricted 

 in the middle, and have at the ends slightly convex, elliptical, articular surfaces. 

 The ventral surface has a median groove for the coccygeal artery. The arch is 

 small and triangular; it is formed of two fiat 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, open above, and soon disappears; the transverse processes 



