118 



PELVIS. 



in the sitting posture. These parts are 

 composed chiefly of hard, laminated bone 

 of dense section. The crest of the ilium 

 is of a spongy structure, as also are the 

 thick masses of bone at the tuberosities 

 of the ilium and ischium, and around the 

 cotyloid cavity. The thinnest parts of the 

 bone are at the bottom of this cavity, and 

 in the centre of the iliac fossa, at which 

 places is so little cancellous structure, that 

 the thin lamina?, forming the whole exterior 

 of the bone, meet each other, and are so thin 

 as to be sometimes translucent. 



The SACKUM (Syn. TO tepov, Gr.; Os sa- 

 crum, basilare, Lat. ; I' Os sacrum, Fr. ; das 

 heilige, or Kreuz Bein, Germ.) is a single 

 pyramidal bone, flattened antero-posteriorly, 

 and placed in the median line, with the apex 

 downwards, the anterior aspect being directed 

 downwards and forwards, and the posterior 

 upwards and backwards. 



Its office is to form a basis of support to 

 the spinal column and its appendages, and, 

 acting like the keystone of an arch between 

 the haunch-bones, to transmit to them its 

 superincumbent weight. It also encloses and 

 protects the terminal divisions of the soinal 

 cord, and transmits them to the inferior parts 

 of the body, and it completes the pelvic ca- 

 vity behind, and affords attachments to the 

 strong muscles of the back. It presents for 

 examination four surfaces ; anterior, posterior, 

 and lateral, a base and an apex. 



The base(jig.78.A) is directed forwards and 

 upwards, and has a general resemblance to the 

 upper surface of a vertebra, the lateral parts 

 exactly corresponding to each other, as in 

 most single bones. In the centre, anteriorly, is 

 a large, plane, oval, articular surface (1) for 

 nbro-cartilaginous union with the last lumbar 

 vertebra, of which the long diameter is placed 

 transversely, and about two inches long. On 

 each side of this surface extends a large, fan- 

 shaped, flattish mass of bone (2), spreading 

 externally, with a rounded anterior border, 

 arching laterally, and forming part of the brim 

 of the true pelvis. On the posterior border 

 is seen a rounded prominence. At the junc- 

 tion of the lateral masses with the oval arti- 

 cular surface posteriorly, two articular pro- 

 cesses (a, a') project upwards and backwards to 

 articulate with those of the last lumbar ver- 

 tebra. Their roots are marked anteriorly bv 

 deep grooves, in which lay the last lumbar 

 nerves. Their articular facets are slightly con- 

 cave, circular, Vertically placed, and directed 

 backwards and inwards, and receive between 

 them their counterparts on the last lumbar ver- 

 tebra. Springing from the roots of these pro- 

 cesses, two long lanrince (1 n) pass backward 

 and inwards with much downward inclination 

 to meet each other in a stunted median uplnous 

 process (2 B), the commencement of the sa- 

 cral crest. They correspond to the laminze 

 and spines'of the true vertebrae. The foramen 

 (3 B) enclosed by them is triangular in shape, 

 and of considerable size, and is the superior 

 opening of the sacral canal. 



The apex (3 A) is a small oval, horizontal 



articular surface placed in the median Hne 

 with its long diameter half an inch in extent, 

 and directed transversely. To it is articulated 

 the base of the coccyx. 



The anterior or pelvic surface is smooth 

 and directed forwards and downwards, form- 

 ing the posterior wall of the true pelvis. 

 It is widest above, opposite the lateral 

 masses of the base (e). Alittle below this point 

 it is about three-fourths of an inch nar- 

 rower (6). It then widens again to the 

 extent of nearly half an inch (/), and then 

 gradually tapers to the apex. It is consider- 

 ably arched from side to side, especially at its 

 superior part, where it has a transverse cur- 

 vature, varying from half to three quarters of 

 an inch in central altitude. Longitudinally, 

 also, this surface is curved to a still greater 

 degree, and with greater variations, upon the 

 comparative extent of which, in male and 

 female, anatomists are much disagreed. 



On each side of the median line are four 

 holes, the anterior sacral foramina., separated 

 from each other by three rounded transverse 

 processes of bone about half an inch wide (4), 

 and placed at equal distances of rather more 

 than half an inch from the median line. The 

 two upper holes are of equal size, and much 

 larger than the two lower. Each is con- 

 nected to its fellow on the opposite side by 

 four raised transverse lines (5), which mark 

 the foetal separation of this bone into five ver- 

 tebrae ; and extending outwards and down- 

 wards from each hole is a groove continued 

 obliquely downwards to the borders of the 

 bone (6). Below the last sacral hole, on 

 each side, is a shallow notch, in the outline 

 of the bone (c, c'), which is transformed into 

 a foramen by the attachment of the upper 

 transverse tubercles of the coccyx. There 

 are many openings for nutritive arteries in 

 this surface, directed generally towards the 

 centre of the bone. 



The posterior surface (fg. B) is rough for 

 muscular attachments, and directed upwards 

 and backwards. It is narrower than the op- 

 posing parts of the anterior at the upper 

 part of the bone generally, by rather more 

 than half an inch. According to Mr. Ward, 

 a transverse section of the sacrum, an inch 

 below the base (at the second sacral ver- 

 tebra), shows that in this place the posterior 

 surface is wider than the anterior by three 

 sixteenths of an inch, so that the sacral wedge 

 is here reversed in obliquity, which he con- 

 siders of importance in resisting anterior 

 dislocation of the sacrum. Above this point, 

 the anterior surface is three sixteenths of an 

 inch wider, and below, it resumes its supe- 

 riority in widi.h by four sixteenths. In some 

 cases the back and front are of equal width ; 

 in others the anterior diameter exceeds the 

 posterior throughout. 



Its general curvatures are convex, following 

 the concavities of the anterior surface. In 

 the median line are four spinous processes 

 (2), the first of which has been described with 

 the base, connected by a sharp vertical ridge 

 of bone, and corresponding to the four upper 



