28 OSTEOLOGY, OR ANATOMY OF OSSEOUS SYSTEM 



The Sacrum and the Coccyx 



The sacrum and the coccyx are the result of the fusing of 

 the lower nine vertebrae into two bones, five to make up the 

 sacrum, and four (occasionally five) the coccyx. 



The sacrum is pyramidal, much larger than the coccyx; 

 situated between the iliac bones, articulating above with the 

 fifth lumbar vertebra, below W 7 ith the coccyx. It is much 

 curved on itself, with the concavity in front. It presents 

 an anterior or ventral and a posterior or dorsal surface, two 

 lateral masses, a base, an apex, and a central canal for descrip- 

 tion. 



The anterior or ventral surface looks considerably downward, 

 forming a projection with the last lumbar, the sacrovcrtcbral 

 angle (about 120 degrees). This surface is concave from above 

 down and less so from side to side, and is crossed by four hori- 

 zontal ridges, indicating the union of five vertebrae. At the 

 ends of the ridges are four (interior sacral foramina, which 

 lead externally into grooves on the lateral masses. 



The two rows of foramina are approximately vertical, only 

 approaching the median line slightly below, as the widths 

 of the bodies are equal. They are directed forward and slightly 

 outward, and diminish in size from above downward. The 

 anterior branches of the sacral nerves pass through them. 



The posterior or dorsal surface looks up and back, is convex 

 and rough, and along the median line are three or four small 

 spinous processes, more or less connected, forming a ridge; 

 the upper is prominent and usually quite distinct from the 

 others. Below the ridge is an inverted V- or U-shaped open- 

 ing into the central canal. It is bounded by the imperfect 

 laminse of the fourth and fifth sacral, and by the inferior articu- 

 lar processes of the last sacral, which are prolonged down into 

 the sacral cornua to meet corresponding ones from the coccyx. 

 On each side of the median ridge the united lamina 3 are hollowed 

 into the sacral yroorr, a continuation of the vertebral groove 

 above, giving origin to a part of the erector spinre; next exter- 

 nally is a row of tubercles representing articular and mammillary 

 processes; next the four posterior sacral foramina, opposite 

 to, but smaller than, the anterior. They transmit the posterior 

 branches of the sacral nerves and correspond to the spaces 



