THE SACRUM 



39 



the third and fourth, and then cHniinishes to the last, which is the shortest. The 

 first one or two usually curve somewhat backward, the last two decidedly forward. 

 Those of the fifth have an oval concave facet on the inner part of the posterior border 

 for articulation with the sixth process; the latter has a corresponding convex facet 

 on the anterior border, and a larger concave surface on the posterior border for 

 articulation with the wing of the sacrum. Sometimes the fifth process has a small 

 surface for articulation with the fourth. The inner part of the sixth process is 

 thick, the outer part thinner, narrower, and curved forward. The inner part of 

 the fifth is also somewhat thickened. 



The spinous processes resem})le tho.se of the last two thoracic vertebra. 

 They are usually al)out ('(jual in height, but minor differences are common, and the 

 width diminishes in tlie la.st three. 



Development. — This is similar to that of the thoracic vertebrae. The extremi- 

 ties of tiie transverse processes remain cartilaginous for some time after ossification 

 is otherwise complete. 



The transverse processes of this region are considered equivalent to the proper transverse 

 process + the co.stal element, and hence the distinctive term processus lateraUs (s. costarius) 

 has been proposed. The occurrence of a lunihar rih in connection with the transverse process of 

 the first lumbar is not rare. Reduction of the number to five has been observed frequently, and 



Spine 



Mammillary processes -=^\ 

 Anterior a r lieu la r proeesses 



Transverse 



process 



Ventral spine 



Fig. is. — Second Lumbar Vkrtkbra or Uorsk, Anterior View. (After Schmaltz, Atlas d. Anat. d. Pferdes.) 



may or may not be compensated by an additional thoracic vertebra. This variation is not more 

 common in certain races as Sanson and others have maintained. Very few cases are recorded of 

 seven lumbar vertebra' — especially with the normal thoracic number. An anomalous vertebra 

 with mixed thoracic and lumbar characters sometimes occurs at the junction of the two regions. 



THE SACRUM 



The sacrum (Os sacrum) is formed by the fusion of five vertebrae usually, and 

 is conveniently described as a single bone. It is triangular in form and is wedged 

 in between the ilia, with which it articulates very firmly on each side. Its long 

 axis is gently curved, and slightly oblique, so that the posterior end is a little higher 

 than the anterior. It presents two surfaces, two borders, a base, and an apex. 



The dorsal surface presents centrally the five sacral spines, which are directed 

 upward and backward, and have (with the exception of the first) tuberous summits 

 which are sometimes bifid. 



The first spine is relatively thin and narrow, and is not so high as the internal angle of the 

 ilium. The second is the longest, and the height diminishes rapidly to the last. The bases of the 

 spines are often fused in old subjects. 



On either side of the spines is a groove, in which are the four dorsal sacral 



