The posterior triangle is bounded laterally by the great sacro-sciatic ligaments, and in the re cent state by the edge of the gluteus maximus muscle also. The coccyx usually protrudes forwards so much that the area of the posterior triangle is less than that of the anterior, not withstanding that the base of each of them is represented by the same line. This posterior triangle contains the anus with the inferior por tion of the rectum, &c., and is usually called the anal division of the perineum.
It should be borne in mind that the measure ments of the inferior outlet of the pelvis inay present considerable variations in different sub jects, and that the operator may be obliged to modify the length and the direction of his inci sions in lithotomy to suit such eases. 11.I. Du puytren, for example, in twenty-three subjects which he examined, found the distance inter mediate between the tuberosities of the ischia to vary from two inches to three and a-half inches; and .51. Velpeau, who measured forty subjects, observed in one case these processes to be but an inch and three quarters asunder, whilst in another they were four inches apart.
In order to perform successfully many of the operations in this region the surgeon requires an accurate knowledge of the axes of the pelvis, and to study the modifications which these imaginary lines exhibit in childhood and old age as compared with adult life. In the full grown male the axis of the superior aperture of the true pelvis takes a direction from the vici nity of the umbilicus downwards and back wards to the coccyx, whilst the axis of the infe rior aperture passes upwards and slightly back wards through the mid space between the tuberosities of the ischia to the promontory of the sacrum ; these two lines intersect each other in the pelvic cavity, forming an angle slightly obtuse and salient posteriorly : the axis of the true pelvis (or in other words a line pass ing through the centres of the upper and lower apertures- respectively) is therefore a curved line concentric with the curvature of the sacrum, and having its concavity directed forwards and downwards towards the symphysis pubis.
During childhood the true pelvis is imper fectly developed ; ithas but little depth, and its capacity is so limited that the viscera which occupy the pelvis of the adult are mostly con tained in the abdominal cavity of the infant.
The straits of the pelvis have each of them likewise an aspect different from that of the adult; in the child the superior aperture looks much more directly forwards, and the inferior aperture much more backwards than they do after puberty. These peculiarities are inherent in the infantile pelvis : they are independent of any inclination in the vertebral column, and they must therefore modify considerably the direction of the pelvic axes during the early years of growth.
In the old subject again the superior aperture is once more directed forwards as in the infant, . vvhilst the inferior aperture inclines backwards ; at this period of life, however, the change in the pelvis arises not from any intrinsic alteration in its bony parietes, but simply from the senile curvature of the spine above, added to the ha bitual flexion of the hip and knee-joints below, so constantly observed in the aged individual.
To understand the course of the rectum and the urethra, as well as the relations of the base of the bladder, the anatomist must study these parts within the pelvis, since it is impossible to display them satisfactorily in the ordinary dissection of the perineum ; much assistance may be derived from preparations affording a side view of the pelvic viscera, and one of the most useful is an antero-posterior section carried through the middle line and dividing the ure thra, prostate gland, bladder, and rectum, after these organs have been moderately dis tended and hardened by alcohol.
REcrinst.—The portion of intestine which belongs to this region commences at the great cul-de-sac of the peritoneum and terminates at the anus. It is perfectly devoid of any serous investment, and presents considerable varieties in size and direction in different subjects. The age and the habits of the individual are found to exert a remarkable influence upon its course and dimensions, and accidental variations in the line of reflection of the serous membrane, which create corresponding changes in the depth of the perineum, are occasionally oh. served even in the adult, and ought to be taken into account by the operator.