they pass inwards through the fat of the ischio-rectal fossa to supply the lower end of the rectum and the muscles in connection with it, as well as the skin around the anus. They anastomose with the corresponding arteries of the opposite side, and with branches from the middle and superior hæmorrhoidal arteries. They likewise send a few twigs round the lower border of the gluteous maximus, in company with the perforating cutaneous nerve to supply the skin in the lower part of the buttock.

The Inferior Hæmorrhoidal Nerve accompanies the vessels of the same name. It may proceed directly from the sacral plexus, but more frequently it is a branch of the internal pudic nerve. Perforating the inner wall of Alcock's canal, it enters the ischio-rectal fossa, and then it breaks up into muscular, cutaneous, and communicating branches. The muscular twigs supply the external sphincter: the cutaneous offsets are given to the skin which surrounds the anus, while the communicating filaments pass forwards to join the long pudendal nerve and the superficial perineal nerves.

Perineal Branch of Fourth Sacral Nerve. This small nerve enters the ischial-rectal fossa by piercing the coccygeus muscle at the side of the coccyx. It is distributed to the skin between the anus and coccyx, and to the external sphincter muscle.

Ano-coccygeal Body.-An indefinite mass of muscular "and fibrous tissue which lies between the tip of the coccyx and the anus receives the name of ano-coccygeal body. It is best seen in sections through the pelvis, and it requires notice on account of the support which it gives to the rectum in front of the coccyx. The muscular tissue which enters into its constitution belongs to the levator ani and the external and internal sphincter muscles (Symington).

UROGENITAL TRIANGLE.

The superficial fascia in this locality has already been

studied. The following is a list of the structures which
still require to be examined:-
:-

1. The superficial perineal vessels and nerves.
2. The long pudendal nerve.

3. The root of the penis The bulb and the crura.

4. The superficial perineal muscles.

5. The triangular ligament.

a. Transversus perinci.

b. Ejaculator urinæ.

c. Erector penis.

6. The internal pudic vessels and nerve and their branches.
7. The compressor urethra muscle.

8. Cowper's glands.

9. The membranous portion of the urethra.

10. The deep layer of the triangular ligament (i.c. the parietal pelvic fascia opposite the pubic arch).

Superficial Perineal Vessels and Nerves.-The superficial perineal vessels and nerves must now be followed out. There are two arteries and three nerves to be looked for :

Arteries.

1. Superficial perineal artery.

2. Transverse perineal artery.

1. Posterior or external superficial perineal nerve. Nerves. 2. Anterior or internal superficial perineal nerve.

3. The long pudendal nerve or the nerve of Soemmerring.

The superficial perineal artery, a branch of the pudic, first pierces the inner wall of Alcock's canal, and then the base of the triangular ligament, so as to gain the interior of the perineal pouch of fascia. It now crosses the transversus perinei muscle, and is continued forwards in the interval between the ejaculator urinæ and erector penis to the scrotum, to the dartos muscle and integuments of which it is distributed in the form of numerous long, slender branches. Before it reaches the scrotum, it supplies twigs to the superficial perineal muscles. It is accompanied by the superficial perineal nerves.

The transverse perineal artery is a small vessel which usually springs from the pudic by a common root of origin with the preceding. It pierces the base of the triangular ligament, and, gaining the surface of the transversus

perinei muscle, proceeds transversely inwards to the interval between the rectum and the bulb, where it ends by supplying the parts in this locality, and by anastomosing with the corresponding vessel of the opposite side.

The posterior superficial perineal nerve, a branch of the perineal division of the pudic nerve, has already been seen in the anterior part of the ischio-rectal fossa, where it effects a communication with the inferior hæmorrhoidal It leaves the fossa by piercing the base of the triangular ligament, and is continued forwards with the superficial perineal artery to the scrotum.

nerve.

The anterior superficial perineal nerve, also derived from the perineal part of the pudic nerve, supplies a few twigs to the levator ani, and, piercing the base of the triangular ligament, is prolonged forward with the posterior nerve to the scrotum. In some instances this nerve passes under cover of the transversus perinei muscle.

Dissection. Instead of searching for the long pudendal nerve at the point where it becomes superficial, and then following it towards its distribution, it is much easier to find it after it has entered the perineal pouch of fascia. Here it will be discovered lying in close relation to the two preceding nerves, but to their outer side. Trace it forwards and backwards. The long pudendal communicates with the inferior hæmorrhoidal nerve, and also with the posterior superficial perineal

nerve.

The long pudendal nerve is derived from an altogether different source. It is a branch of the small sciatic nerve, and pierces the deep fascia of the thigh a short distance in front of the tuber ischii, and about an inch and a half to the outer side of the margin of the pubic arch. As it proceeds forwards it inclines inwards, and, piercing the attachment of the superficial fascia to the margin of the pubic arch, it accompanies the other vessels and nerves. to the scrotum, the outer and front part of which it supplies.

Dissection. Divide the superficial perineal vessels and nerves, and throw them aside.

Root of the Penis.—At this stage of the dissection the student should consider the position of the triangular ligament, and the relation which it bears to the root of the penis. The triangular ligament is a strong aponeurotic membrane which stretches across the pubic arch, and subdivides the urogenital portion of the perineum into a superficial and a deep area. The root of the penis is placed altogether in front of it, in the superficial area of this region. With the handle of the knife clear away for a short distance the loose tissue which surrounds the body of the penis. The body of the penis is then seen to consist of three cylindrical masses which are chiefly composed of erectile tissue, and are placed in close apposition with each other. These are the two corpora cavernosa and the corpus spongiosum.

The corpora cavernosa constitute the chief bulk of the organ. They are placed side by side and form the dorsum and sides of the penis. They are partially blended with each other along the middle line, indeed the only surface indication of the double nature of this portion of the penis are two median longitudinal grooves which run one along its upper and the other along its lower aspect. The corpus spongiosum is slender in comparison with the corpora cavernosa, and is lodged in the groove which extends along the lower aspect of these bodies. On account of this, the body of the penis has a somewhat prismatic form. The corpus spongiosum is traversed throughout its whole extent by the urethra.

If these three constituents of the body of the penis be traced backwards, the student will observe that opposite the lower part of the symphysis pubis they separate from each other and become attached to parts in the superficial area of the urogenital triangle. The corpora cavernosa diverge widely from each other, and now they receive the name of the crura of the penis. Each crus is fixed firmly to the corresponding side of the pubic arch by an attachment which extends from the sub-pubic ligament backwards to the tuberosity of the ischium. Close to the point where

it becomes continuous with the corresponding corpus cavernosum it shows a slight dilatation or bulging; from this to its posterior extremity it gradually tapers away. The corpus spongiosum is continued backwards in the middle line of the body to within a short distance of the anus, and it gradually expands so as to form a bulbous posterior extremity. The corpus spongiosum, as it lies in the interval between the diverging crura, is therefore termed the bulb of the penis. The bulb rests upon the superficial aspect of the triangular ligament, and it is firmly

crus.

bulb.

T

FIG. 74.-Diagram of the three parts of the penis, and their relation to the pubic arch and the triangular ligament.

T. Triangular ligament.

bound down to this by an aponeurotic investment, which is prolonged over it from the ligament. The posterior extremity of the bulb is notched in the middle line-an indication of its originally double constitution.

The bulb and the two crura together constitute the root of the penis, and each is provided with a special muscle, which at present hides it from view. Clothing the bulb the student will recognise a bipenniform muscle called the ejaculator urine, whilst moulded upon the surface of