OF THE PELVIS. 27 



five inches to the other. The mobility of the coccyx, and the elas- 

 ticity of the sciatic ligaments, render the antero-posterior diameter 

 capable of being prolonged four, six, eight, or even twelve lines, 

 while the oblique diameters are also evidently capable of being elon- 

 gated; the transverse, on the contrary, I have always found to be 

 a few lines less than four inches. Thus the circumference of the 

 perineal strait should be about twelve inches. 



52. In general, the plane of the inferior strait is inclined slightly 

 upwards, so that the line that represents it crosses that of the supe- 

 rior strait in front of the symphysis of the os pubis; however, it is 

 sometimes found to be horizontal, and even below the level of the 

 coccyx. 



53. The axis of the inferior strait is represented by a straight line 

 drawn from the interior of the pelvis, and cutting the middle of the 

 coccy-pubal diameter at right angles; the upper extremity of this 

 line most commonly rises as high as the sacro-vertebral angle, and 

 sometimes is found to be even parallel with the spinal column, and 

 may approach even nearer to the axis of the superior strait in very 

 many instances, as is proved by the late researches of professor 

 Naegele, and as I have ascertained for myself. 



54. In order to obtain correct notions on this subject, it is neces- 

 sary to suppose the coccyx depressed, as it is by the head of the 

 foetus at the moment of delivery; the posterior extremity of the 

 coccy-pubal diameter will then be found lower than its anterior 

 extremity; and the axis of the strait descends obliquely from behind 

 forwards, under an angle of from fifteen to twenty decrees, passing 

 from the anterior face of the first or second piece of the sacrum, 

 through the middle of the space which separates the anterior parts 

 of the tuberosities of the ischia; it is requisite, further, not to forget 

 that in the living subject, and particularly during the passage of the 

 head, the obliquity of this line is so much increased, that it becomes 

 almost parallel with the plane of the superior strait. 



§• IV. Dimensions of the Excavation. 



55. The anterior wall of the pelvic cavity, opposite to the sym- 

 physis of the pubis, is only eighteen lines high; but more outwardly 

 it is near three inches: the depth of the middle portion of the lateral 

 regions is at least three inches and a half; the posterior wall is at 

 least five along the median line, following the curve of the sacrum; 

 and only four in a straight line, from the promontory to the point of 

 the coccyx. From these differences it is manifest that the inclina- 

 tion of the straits will always be about two inches and a half, and in 

 an inverse proportion, in each case; for the pubis cannot be depress- 



