THE PENIS - 581 
surface is convex; its lower part slopes backward, and presents 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 urethr), 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 skin. This diverticulum is 
filled sometimes with a caseous mass of sebaceous matter and epithelial débris. 
Behind the corona glandis there is a constriction, the collum glandis. It is to be 
noted, however, that this does not indicate the demarcation between the glans and 
corpus penis, since the former extends backward above the corpus cavernosum a 
distance of about four inches (ca. 10 em.), forming the processus dorsalis glandis 
(Fig. 518). 
Structure.—The penis consists essentially of two erectile bodies, the corpus 
cavernosum penis and the corpus cavernosum urethre. 
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 
penis, which is embedded in the ischio-cavernosus muscle. Below the ischial arch 
the crura unite to form the laterally compressed body of the corpus cavernosum; 
this presents ventrally the urethral 
groove (Sulcus urethralis), which con- Dorsum penis 
tains the urethra and corpus caverno- \ 
sum urethrie. Anteriorly the corpus 
cavernosum penis divides into three 
processes—a long central one, which is 
capped by the glans penis, and two 
Trabecule 
Corpus 
short blunt lateral ones. The corpus cavernosum Tunica _ 
cavernosum is enclosed by the tunica penis are 
albuginea, a thick capsule of fibrous 
tissue which contains some elastic 
fibers. Externally the fibers are chiefly ? 
longitudinal; internally they aremainly = C"P¥s STE 
circular and are looser in arrangement. eee a MM Bulbo- 
Numerous trabecule pass inward from ernOSUs 
the tunic and form a framework in the muscle 
interior of the corpus cavernosum. En- 
closed by this framework is the erectile 
tissue, which is readily distinguished Fic. 519.—Cross-section or Bopy oF Pents oF Horse. 
from the fibrous trabecule by its red- 
dish-gray color and softer texture. It is composed largely of strands of unstriped 
muscle, between which there are cavernous spaces (Caverne). These spaces may 
be regarded as greatly enlarged capillaries; they contain blood, are lined with flat 
endothelial cells resting upon a layer of delicate connective tissue, and are directly 
continuous with the veins of the penis. Erection is produced by distention of 
these spaces with blood; at other times the spaces are mere slits. 
Retracior penis muscle 
In man there are two distinct corpora cavernosa, separated by a median septum penis, 
which is complete except in the middle part of the organ, where the septum is composed of vertical 
trabeculz, 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 trabeculze, which 
form an arrangement like the septum pectiniforme of man. 
The corpus cavernosum urethre (or corpus spongiosum) forms a tube around 
the urethra, and is continuous at its anterior end with the glans penis. It forms a 
slight enlargement at the root of the penis, which is termed the bulb (Bulbus 
urethre). In the body of the penis it forms a thinner layer dorsally than on the 
sides and ventrally. The structure of the corpus cavernosum urethrz is somewhat 
