62 



SPECIAL ANATOMY OF THE SKELETON 



bral foramina and vertebral canal. The first segment is the largest ; it resembles 

 the lowermost sacral vertebra, and often exists as a separate piece; the last three, 

 diminishing in size from above downward, are usually blended together so as 

 to form a single bone. The gradual diminution in the size of the pieces gives 

 this bone a triangular form, the base of the triangle joining the apex of the 

 sacrum. It presents for examination an anterior and posterior surface, two 

 borders, a base, and an apex. 



Cornnn. 

 / 



Anterior surface. 



* v 



Posterior surface. 



FIG. 26. Coccyx. 



Surfaces. Anterior Surface. The anterior surface is slightly concave and 

 marked with three transverse grooves, indicating the points of junction of the differ- 

 ent pieces. It has attached to it the anterior sacrococcygeal ligament and Levator 

 ani muscle, and supports the lower end of the rectum. 



Posterior Surface. The posterior surface is convex, marked by transverse 

 grooves similar to those on the anterior surface; and presents on each side a lineal 

 row of tubercles, the rudimentary articular processes of the coccygeal vertebra?. 

 Of these, the superior pair are large, and are called the cornua of the coccyx (cornua 

 coccygea) ; they project upward, and articulate with the cornua of the sacrum, the 

 junction between these two bones completing the fifth posterior sacral foramen for 

 the transmission of the posterior division of the fifth sacral nerve. 



Borders. The lateral borders are thin, and present a series of small emi- 

 nences, which represent the transverse processes of the coccygeal vertebrae. Of 

 these, the first on each side is the largest, flattened from before backward, and often 

 ascends to join the lower part of the thin lateral edge of the sacrum, thus completing 

 the fifth anterior sacral foramen for the transmission of the anterior division 

 of the fifth sacral nerve; the others diminish in size from above downward, and 

 are often wanting. The borders of the coccyx are narrow, and give attachment on 

 each side to the sacrosciatic ligaments, to the Coccygeus muscles in front of the 

 ligaments, and to the Gluteus maximus behind them. 



Base. The base presents an oval surface for articulation with the sacrum. 



Apex. The apex is rounded, and has attached to it the tendon of the external 

 Sphincter muscle. It is occasionally bifid, and sometimes deflected to one or 

 other side. 



Articulation. With the sacrum. 



Attachment of Muscles. To four pairs and one single muscle: on either side, the Coccygeus; 

 behind, the Gluteus maximus and Extensor coccygis, when present; at the apex, the Sphincter 

 ani; and in front, the Levator ani. 



Structure of the Vertebrae. The body is composed of light, spongy, cancellous tissue, 

 having a thin coating of compact tissue on its external surface perforated by numerous orifices 

 of various sizes for the passage of vessels; its interior is traversed by one or two large canals 



