122 MAMMALIAN ANATOMY 



THE SACRUM ^-VARIATIONS IN DEVELOPMENT. 



The spinous process of the third vertebra may be rudimentary or 

 coalesced by its cephalic border with the spinous process of the second 

 vertebra ; rarely the first spinous process is smaller than the second 

 (three cases in thirty-five). 



The cephalic articular processes are either low, with their articular 

 surfaces facing dorsally more than medially, or high, with their surfaces 

 facing directly toward the middle line. The emarginatiou which sepa- 

 rates them may be wide and shallow, or narrow and deep. 



The laminae often exhibit along their lines of coalescence irregular 

 openings or small, definite foramina. 



The outline of the lateral mass varies from almost circular to tri- 

 angular ; hence the shape of the auricular surface also varies ; it may 

 extend, on the narrow lateral border of the sacrum, beyond the first 

 dorsal foramen. 



The ventral surface may be very flat or convex, either trans- 

 versely and in a cephalo-caudal direction, or in one of these directions 

 only. These differences in curvature appear to be independent of 

 sex. 



The sacral foramina vary in size, but no case is noted of the 

 entire absence of any of them. 



VARIATIONS IX THE NUMBER OF VERTEBRAE. 



In a specimen described a as having fourteen thoracic vertebrae, 

 the additional rib 011 each side was small (one to one and a half inches 

 in length), and its point of attachment to the vertebra was similar to 

 that of the rib of the preceding vertebra and to that of the trans- 

 verse process of the succeeding vertebra. The change in the artic- 

 ular processes from the thoracic to the lumbar type was completed 

 between the eleventh and twelfth vertebrae, but the caudal articular 

 processes of the tenth, although of a dorsal type, presented transitional 

 changes. 



I have observed several instances of an increase in the number of 

 the lumbar vertebrae, but the eighth lumbar was always attached, in a 

 greater or less degree, to the sacrum, and its coalesced transverse 

 process or processes assisted in the formation of the sacro-iliac articu- 



1 Struthers, Jour. Anat. and Phys., 1875, p. 64. 



