THE PENIS 



495 



The glaiis penis is the enlarged free end of the organ. It is marked off from 

 the body by a faintly marked neck (C'ollum glandis). In front of this is a promi- 

 nent circular ridge, the corona glandis, which is notched below. The base of the 

 glans is rounded and extends further l^ackward 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 urethrae), 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 b}- thin black skin. This diverticulum is filled some- 

 times with a caseous mass of sebaceous matter and epithelial debris. 



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 l)ulk 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-ca vernosus 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 sjiongiosum. An- 



Dorsum penis 



L-orpus 

 cavernosum 



Corpus 

 spongiosum 



Trabcculoe 



Tunica 



albuginea 



Urethra 



— Bulho- 



carernosus 

 muscle 



Retractor penis muscle 

 Fig. 396. — Cross-skction of Body of Pevis of Horse. 



teriorl}', the corpus cavernosum divides 

 into three processes, a long central one, 

 which is capped by the glans penis, 

 and two short ijlunt lateral ones. The 

 corpus caN'crnosum is inclosed ])y the 

 tunica albuginea, a thick capsule of 

 fibrous ti.ssue which contains many 

 elastic fibers and some unstriped 

 nmscle. Externally the fibers are 

 chiefly longitudinal; internally they 

 are mainly circular and are looser in 

 arrangement. Numerous trabeculae 

 pass inward from the tunic antl form 

 a sponge-like framework in the in- 

 terior of the corpus cavernosum, 

 which is thus divided into numerous 

 spaces (Cavernae). These spaces may 

 be regarded as greatly enlarged capil- 

 laries; 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. 



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 trabeculce, be- 

 tween which are sUt-Uke intervals; through the latter the blood-spaces of the two corpora caver- 

 nosa communicate. In the horse no distinct septum exists except near the root, but in the proxi- 

 mal and distal parts of the corpus cavernosum there are vertical trabeculse. 



The corpus spongiosum or corpus cavernosum urethrae forms a tube around 

 the urethra, and is nmch 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 urethrae) 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. 



