The =penis=, the male organ of copulation, is composed essentially of
erectile tissue, and incloses the extrapelvic part of the urethra. It
extends from the ischial arch forward between the thighs on to the
umbilical region of the abdominal wall. It is supported by the fascia
penis and the skin, and its prescrotal portion is situated in a
cutaneous pouch, the prepuce or sheath. It is cylindrical in form, but
much compressed laterally in the greater part of its extent.
[Illustration:
FIG. 395.—PENIS OF HORSE, LATERAL VIEW.
]
In the quiescent state it is about 20 inches (ca. 50 cm.) long; of
this, about 6 to 8 inches (ca. 15 to 20 cm.) is free in the prepuce.
In erection it increases 50 per cent. or more in length.
It may be divided into a root, a body, and a terminal enlargement, the
glans.
The =root= (Radix penis) is attached to the lateral parts of the ischial
arch by two =crura=, which converge and unite below the arch (Fig. 452).
The urethra passes over the ischial arch between the crura, and curves
forward to become incorporated with the penis.
The =body= (Corpus penis) begins at the junction of the crura and
constitutes the bulk of the organ. At its origin it is attached to the
symphysis ischii by two strong flat bands, the =suspensory ligaments= of
the penis (Ligamenta suspensoria penis), which blend with the tendon of
origin of the graciles muscles (Figs. 451, 456). This part of the penis
is flattened laterally for the most part, but becomes rounded and
smaller anteriorly. It presents four surfaces. The =dorsal surface= or
=dorsum penis=, is narrow and rounded; on it are the dorsal arteries and
nerves of the penis and a rich venous plexus. The =ventral= or =urethral
surface= (Facies urethralis) is rounded, and along it runs the urethra,
embedded in the deep =urethral groove= of the corpus cavernosum. The
=lateral surfaces= are high and flattened, except anteriorly, where they
are lower and rounded.
The =glans penis= is the enlarged free end of the organ. It is marked
off from the body by a faintly marked =neck= (Collum glandis). In front
of this is a prominent circular ridge, the =corona glandis=, which is
notched below. The base of the glans is rounded and extends further
backward dorsally than ventrally; it is marked in its lower part by a
deep depression, the =fossa glandis=, in which the urethra protrudes for
about an inch (ca. 2.5 cm.) as a free tube, the =urethral process=
(Processus urethræ), covered by a thin integument. The urethra is thus
surrounded by a circular fossa, which opens superiorly into the
=urethral sinus=, a bilocular diverticulum lined by thin black skin.
This diverticulum is filled sometimes with a caseous mass of sebaceous
matter and epithelial débris.
=Structure.=—The penis consists essentially of two erectile bodies, the
corpus cavernosum penis and the corpus spongiosum.
The =corpus cavernosum penis= forms the greater part of the bulk of the
penis except at its free extremity. It arises from each side of the
ischial arch by a =crus=, which is embedded in the ischio-cavernosus
muscle. Below the ischial arch the crura fuse completely to form the
laterally compressed body of the corpus cavernosum; this presents
ventrally the =urethral groove= (Sulcus urethralis), which contains the
urethra and corpus spongiosum. Anteriorly, the corpus cavernosum divides
into three processes, a long central one, which is capped by the glans
penis, and two short blunt lateral ones. The corpus cavernosum is
inclosed by the =tunica albuginea=, a thick capsule of fibrous tissue
which contains many elastic fibers and some unstriped muscle. Externally
the fibers are chiefly longitudinal; internally they are mainly circular
and are looser in arrangement. Numerous =trabeculæ= pass inward from the
tunic and form a sponge-like framework in the interior of the corpus
cavernosum, which is thus divided into numerous spaces (Cavernæ). These
spaces may be regarded as greatly enlarged capillaries; they contain
blood, are lined with flat endothelial cells, and are directly
continuous with the veins of the penis. Their walls are composed very
largely of unstriped muscle. Erection is produced by distention of these
spaces with blood; at other times the spaces are mere slits.
[Illustration:
FIG. 396.—CROSS-SECTION OF BODY OF PENIS OF HORSE.
]
In man there are two distinct corpora cavernosa, separated by a
complete septum penis, except in the middle part of the organ, where
the septum is composed of vertical trabeculæ, between which are
slit-like intervals; through the latter the blood-spaces of the two
corpora cavernosa communicate. In the horse no distinct septum exists
except near the root, but in the proximal and distal parts of the
corpus cavernosum there are vertical trabeculæ.
The =corpus spongiosum= or =corpus cavernosum urethræ= forms a tube
around the urethra, and is much enlarged at its free end to form the
bulk of the glans penis; throughout the remainder of its course it is
practically uniform in size, and does not form any very distinct bulb
(Bulbus urethræ) at its origin, as in man and many animals. The glans
has been described in part; it should be noted that the corpus
spongiosum extends backward dorsally a distance of about four inches
(ca. 10 cm.) on the middle process of the corpus cavernosum penis,
forming the processus dorsalis glandis. The urethral process is covered
by a thin layer of corpus spongiosum.
In structure the corpus spongiosum has a general resemblance to the
corpus cavernosum, but the trabeculæ are finer and more elastic. In the
glans the trabeculæ are highly elastic, and the spaces are large and
very extensible; the latter are continuous at the extremity of the
processus dorsalis with large veins on the dorsum penis. There is a
partial septum glandis. The skin covering the glans is thin, destitute
of glands, and richly supplied with nerves and special nerve-endings.
=Vessels and Nerves.=—The penis is supplied with blood by three
=arteries=, viz., the =internal pudic=, =obturator=, and =external
pudic=. The =veins= form a rich plexus on the dorsum and sides of the
penis, which is drained by the =external pudic= and =obturator veins=;
from the root the blood is carried by the =internal pudic veins=. The
=lymphatics= run with the veins and go to the inguinal and lumbar
glands. The =nerves= are derived chiefly from the =pudic nerves= and the
=pelvic plexus= of the sympathetic.