roPVLATORY ORdAN^ 249 



a subjacent region is composed of denser connective tissue and 

 skeletal muscle. During the breeding season, when co])ulati()n is 

 to take place, the vascular spaces become engorged with l)lood 

 and convert it into erectile tissue that is thrust out from the cloacal 

 opening. Between breeding seasons this structure lies colla])sed 

 within the cloaca. 



The monotreme has a cloacal penis somewhat like that of re])tiles 

 and birds, except that the urinary passage, the urethra, runs through 

 its center. This penis lies retracted in a cloacal sheath except during 

 the breeding season. 



Penis of Mammals. -In the body, or shaft, of the penis, masses 

 of mesenchyme tissue give rise to corpora cavernosa or erectile 

 tissue and also the cartilage or bone which forms in some species. 

 The urethra passes only a short distance from the bladder before 

 becoming associated with the ducts and glands of the genital system. 

 Xear the bladder it has a transitional epithelial lining, but this 

 often changes to stratified columnar further along and then to strati- 

 fied squamous at the o])ening. Three regions of the urethra ha^-e 

 been separated. The prostatic portion extends from the neck of 

 the bladder through the prostate gland, where ejaculatory ducts 

 usually join it. The membranous, or middle part, is short, extend- 

 ing from the i)rostate region to the beginning of the corpora caver- 

 nosa of the penis. The cavernosa portion extends through the length 

 of the penis. Along the course of the urethra are glandular out- 

 pocketings of the dorsal and lateral walls, already described, embed- 

 ded in the fibroelastic connective tissue, with varying amounts of 

 smooth muscle also represented. 



A cross-section through the body of the penis shows that the 

 major portion of it is formed by the corpora cavernosa. In man, 

 two corpora cavernosa penis occupy the upper portion, but in other 

 mammals they usually fuse to form an unpaired U-shaped mass. 

 In man a third, corpus cavernosum urethra, surrounds the urethra 

 and is located ^•entral to the other two (corpora cavernosa penis). In 

 many mammals the urethral ca\'ernosa tissue is poorly represented. 

 (Fig. 154.) These corpora constitute the erectile tissue of the penis. 

 Among the rodents, the vas deferens may not join the urethra until 

 they reach the tij) of the penis. 



The corpora cavernosa are composed of a vascular sponge-like 

 network with irregular venous sinuses capable of being filled with 

 blood and distended under considerable pressure. Each of these 

 bodies is surrounded by a thick fibrous membrane, the tunica 



