The region which extends from the umbilicus to the point of the coccyx is marked upon the cutaneous surface by a central raphe dividing the hypogastrium, the penis, the scrotum, and the perinaeum respectively into equal and similar sides. The umbilicus is a cicatrix formed after the metamorphosis of a median foetal structure—the placental cord, &c. In the normal form, the meatus urinarius and the anus coincide with the line of the median raphe, and signify omissions at stated intervals along the line of central union. When between these intervals the process of union happens likewise to be arrested, malformations are the result; and of these the following are examples:—Extrusion of the bladder at the hypogastrium is caused by a congenital hiatus at the lower part of the linea alba, which is in the median line; Epispadias, which is an urethral opening on the dorsum of the penis; and Hypospadias, which is a similar opening on its under surface, are of the same nature—namely, omissions in median union. Hermaphrodism may be interpreted simply as a structural defect, compared to the normal form of the male, and as a structural excess compared to that of the female. Spina bifida is a congenital malformation or hiatus in union along the median line of the sacrum or loins. As the process of union along the median line may err by a defect or omission, so may it, on the other hand, err by an excess of fulfilment, as, for example, when the urethra, the vagina, or the anus are found to be imperforate. As the median line of union thus seems to influence the form of the hypogastrium, the genitals, and the perinaeum, the dissection of these parts has been conducted accordingly.
By removing the skin and subjacent adipose membrane from the hypogastrium, we expose the superficial fascia. This membrane, E E E*, Fig. 1, Plate 50, is, in the middle line, adherent to B, the linea alba, and thereby contributes to form the central depression which extends from the navel to the pubes. The adipose tissue, which in some subjects accumulates on either side of the linea alba, renders this depression more marked in them. At the folds of the groin the fascia is found adherent to Poupart’s ligament, and this also accounts for the depressions in both these localities. From the central linea alba to which the fascia adheres, outwards on either side to the folds of both groins, the membrane forms two distinct sacs, which droop down in front, so as to invest the testicles, E**, and penis in a manner similar to that of the skin covering these parts. As the two sacs of the superficial fascia join each other at the line B, coinciding with the linea alba, they form by that union the suspensory ligament of the penis, which is a structure precisely median.
The superficial fascia having invested the testicles each in a distinct sac, the adjacent sides of both these sacs, by joining together, form the median septum scroti, E, Fig. 2, Plate 50. In the perinaeum, Fig. 1, Plate 51, the fascia, A, may be traced from the back of the scrotum to the anus. In this region the membrane is found to adhere laterally to the rami of the ischium and pubes; whilst along the median perinæal line the two sacs of which the membrane is composed unite, as in the scrotum, and form an imperfect septum. In front of the anus, beneath the sphincter ani, the fascia degenerates into cellular membrane, one layer of which is spread over the adipose tissue in the ischio-rectal space, whilst its deeper and stronger layer unites with the deep perinæal fascia, and by this connexion separates the urethral from the anal spaces. The superficial fascia of the hypogastrium, the scrotum, and the perinaeum forming a continuous membrane, and being adherent to the several parts above noticed, may be regarded as a general double sac, which isolates the inguino-perinæal region from the femoral and anal regions, and hence it happens that when the urethra becomes ruptured, the urine which is extravasated in the perinaeum, is allowed to pass over the scrotum and the abdomen, involving these parts in consequent inflammation, whilst the thighs and anal space are exempt. The tunicae vaginales, which form the immediate coverings of the testicles, cannot be entered by the urine, as they are distinct sacs originally protruded from the abdomen. It is in consequence of the imperfect state of the inguino-perinæal septum of the fascia, that urine effused into one of the sacs is allowed to enter the other.
Like all the other structures which join on either side of the median line, the penis appears as a symmetrical organ, D D, Fig. 2, Plate 50. While viewed in section, its two corpora cavernosa are seen to unite anteriorly, and by this union to form a septum “pectiniforme;” posteriorly they remain distinct and lateral, F F, Fig. 2, Plate 51, being attached to the ischio-pubic rami as the crura penis. The urethra, B, Fig. 2, Plate 50, is also composed of two sides, united along the median line, but forming between them a canal by the cleavage and partition of the urethral septum. All the other structures of the perinaeum will be seen to be either double and lateral, or single and median, according as they stand apart from, or approach, or occupy the central line.
The perinaeum, Figs. 1, 2, Plate 51, is that space which is bounded above by the arch of the pubes, behind by C, the os coccygis, and the lower borders of, I I, the glutaei muscles and sacro-sciatic ligaments, and laterally by D D, the ischiatic tuberosities. The osseous boundaries can be felt through the integuments. Between the back of the scrotum and the anus the perinaeum swells on both sides of the raphe, A B, Fig. 3, Plate 50, and assumes a form corresponding with the bag of the superficial fascia which encloses the structures connected with the urethra. The anus is centrally situated in the depression formed between D D, the ischiatic tuberosities, and the double folds of the nates.
The perinaeum, Fig. 3, Plate 50, is, for surgical purposes, described as divisible into two spaces (anterior and posterior) by a transverse line drawn from one tuber ischii, D, to the other, D, and crossing in front of the anus. The anterior space, A D D, contains the urethra; the posterior space, D D C, contains the rectum. The central raphe, A B C, traverses both these spaces. The anterior or urethral space is (while viewed in reference to its osseous boundaries) triangular in shape, the apex being formed by the pubic symphysis beneath A, whilst two lines drawn from A to D D, would coincide with the ischio-pubic rami which form its sides. The raphe in the anterior space indicates the central position of the urethra, as may be ascertained by passing a sound into the bladder, when the shaft of the instrument will be felt prominently between the points A B. Behind the point B, the sound or staff sinks deeper in the perinaeum as it follows the curve of the urethra towards the bladder, and becomes overlaid by the bulb, &c.
The ischiatic tuberosities, D D, Fig. 3, Plate 50, are, in all subjects, sufficiently prominent to be felt through the integuments, &c.; and the line which, when drawn from one to the other, serves to divide the two perinæal spaces, forms the base of the anterior one. In well-formed subjects, the anterior space is equiangular, the base being equal to each side; but according as the tuberosities approach the median line, the base becomes narrowed, and the triangle is thereby rendered acute. These circumstances influence the direction in which the first incision in the lateral operation of lithotomy should be made. When the tuberosity of the left ischium stands well apart from the perinæal centre, the line of incision, B E, Fig. 3, Plate 50, is carried obliquely from above downwards and outwards; but in cases where the tuberosity approaches the centre, the incision must necessarily be made more vertical. The posterior perinæal space may be described on the surface by two lines drawn from D D, the ischiatic tuberosities, to C, the point of the coccyx, whilst the transverse line between D and D bounds it above.
By removing the integument and superficial fascia, we expose the superficial vessels and nerves, together with the muscles in the neighbourhood of the urethra and the anus. The accelerator urinae, E, Fig. 2, Plate 51, which embraces the urethra, and the sphincter ani, B C, which surrounds the anus, H, occupy the median line, and are divided each into halves by a central tendon, E B C, which traverses the perinaeum from before backwards, to the point of the coccyx. On either side of the anus, in the ischio-rectal space, D D, Fig. 1, Plate 51, is found a considerable quantity of granular adipose tissue, traversed by the inferior haemorrhoidal arteries and nerves-branches of the pudic artery and nerve.
In front of the anus are seen two small muscles (transversae perinaei), G G, Fig. 2, Plate 51, each arising from the tuber ischii of its own side, and the two becoming inserted into, B, the central tendon. These transverse muscles serve to mark the boundary between the anterior and posterior perinæal spaces. Behind each muscle is found a small artery, crossing to the median line. The left transverse muscle and artery are always divided in the lateral operation of lithotomy. On the outer sides of the anterior perinæal space are seen the erectores penis muscles, F F, overlaying the crura penis. Between each muscle and the accelerator urinae, the superficialis perinaei artery and nerve course forwards to the scrotum, &c.
The perinæal muscles having been brought fully into view, Plate 52, Fig. 1, their symmetrical arrangement on both sides of the median line at once strikes the attention. On either side of the anterior space appears a small angular interval, L, formed between B, the accelerator urinae, D, the erector penis, and E, the transverse muscle. Along the surface of this interval, the superficial perinæal artery and nerve are seen to pass forwards; and deep in it, beneath these, may also be observed, L, the artery of the bulb, arising from the pudic, and crossing inwards, under cover of the anterior layer of the membrane named the deep perinæal fascia. The first incision in the lateral operation of lithotomy is commenced over the inferior inner angle of this interval.