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 urethree), 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 debris. 

 Behind the corona glandis there is a constriction, the coUum 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 cm.), forming the processus dorsalis glandis 

 (Fig. 518). 



Structure. — The penis consists essentially of two erectile bodies, the corpus 

 cavernosum penis and the corpus cavernosum urethras. 



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 cms 

 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 



Dorsum peyiis 



Corpus 

 carer nosum 



Corpus 

 cavernosum 

 urethras 



Trabecules 



Tunica 



albuginea 



Urethra 



Bulbo- 



cavernosus 

 muscle 



Retractor penis muscle 

 Fig. 519. — Cboss-seotion of Body of Penis of Hohse. 



groove (Sulcus urethralis), which con- 

 tains the urethra and corpus caverno- 

 sum urethrae. Anteriorly the corpus 

 cavernosum penis 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 enclosed by the tunica 

 albuginea, a thick capsule of fibrous 

 tissue which contains some elastic 

 fibers. Externally the fibers are chiefly 

 longitudinal; internally they are mainly 

 circular and are looser in arrangement. 

 Numerous trabeculae pass inward from 

 the tunic and form a framework in the 

 interior of the corpus cavernosum. En- 

 closed by this framework is the erectile 

 tissue, which is readily distinguished 

 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 (Caverns). 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. 



In man there are two distinct corpora cavernosa, separated by a median septum penis, 

 which is complete except in the middle part ot the organ, where the septum is composed of vertical 

 trabecute, 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 trabecula;, which 

 form an arrangement like the septum pectiniforme of man. 



The corpus cavernosum urethras (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 

 urethra;). In the body of the penis it forms a thinner layer dorsally than on the 

 sides and ventrally. The structure of the corpus cavernosum urethra is somewhat 



