96 OSTEOLOGY. 



which they are serially homologous, and are to be regarded as representing the 

 costal elements. Placed on their bases posteriorly, and just lateral to and inferior to 

 the superior articular processes, are the small accessory processes, which are in series 

 with the inferior tubercles of the lower thoracic vertebrae. The superior articular 

 processes are stout, oval, curved plates of bone, fused in front with the roots and 

 laminae, and having their concave articular surfaces vertical and turned medially. 

 Laterally, and on their posterior edge, the bone rises in the form of an elongated 

 oval tubercle, the processus mamillaris (mamillary process); these are in 

 correspondence with the superior tubercles of the lower thoracic transverse 

 processes. 



The inferior articular processes lie on either side of the root of the spinous 

 process, supported on the inferior margin of the laminae. Their articular surfaces, 

 oval in outline, convex from side to side, and plane from above downwards, are 

 turned laterally. The inferior articular processes are much closer together than the 

 superior ; so that when the vertebras are articulated the superior articular processes 

 of the lower vertebra embrace the inferior articular processes of the higher vertebra. 



The fifth lumbar vertebra is characterised by the size of its body, which is 

 the largest of all the vertebrae. Further, the inferior surface of the body is cut 

 away at the expense of its posterior part : hence the thickness of the body in 

 front much exceeds the vertical diameter behind. By its articulation with 

 the first sacral segment the inferior border of the body of this bone assists 

 in the formation of the sacro-vertebral angle. The transverse process is pyra- 

 midal in form, and stouter than those of the other lumbar vertebrae. It arises 

 by a broad base from the side of the back of the body, as well as from the 

 pedicle, and is directed laterally and a little backwards and upwards. Its upper 

 surface is slightly grooved by the superior intervertebral notch. A deep notch 

 separates it posteriorly from the superior articular processes, which are less 

 in-turned than in the other members of the series, their articular surfaces being 

 directed more backwards than inwards, and displaying less concavity. The inferior 

 articular processes are further apart than is the case with the other members of 

 the series ; they lie in line with the superior. The spinous process is shorter and 

 narrower than the other lumbar spines, particularly so in the female. The 

 vertebral canal is somewhat compressed at its lateral angles. 



THE FALSE OR FIXED VERTEBRAE. 

 Os Sacrum. 



The sacrum, of roughly triangular shape, is formed normally by the fusion of 

 five vertebrae. The anterior surface of the bone is slightly hollow from side to side 

 and concave from above downwards, the curve being usually most pronounced 

 opposite the third sacral segment. The central part corresponds to the bodies 

 of the sacral vertebrae, the lines of fusion of which are indicated by a series of 

 four parallel ridges which cross the median part of the bone at gradually diminish- 

 ing intervals from above downwards; on each side these ridges disappear on 

 the medial walls of the four anterior sacral foramina. The size of these holes 

 decreases from above downwards. The upper and lower border of each foramen 

 is formed by a stout bar of bone, of which there are five on each side, corre- 

 sponding in number with the vertebrae present. These unite laterally so as 

 to form the pars lateralis (O.T. lateral mass), thus enclosing the foramina to the 

 lateral side, though there the edge is not abrupt, but sloped so as to pass gradually 

 into the canal. The large anterior rami of the sacral nerves pass through these 

 foramina and occupy the shallow grooves. The bone is broadest across the first 

 sacral vertebra, tends to narrow opposite the second, and again usually increases 

 in width opposite the third. When this condition is well marked, the edge has 

 a notched appearance (sacral notch) which assists in the interlocking of the sacro- 



