MALE GENITAL SYSTEM: INTERNAL ORGANS 381 



Dorsally within the substance of the ridge the smooth muscles spread out to form a 

 semicircular layer of longitudinal fibres underneath the fibrous investment of the canal. 



The prostata is surrounded throughout its length by a thick layer of circular striated 

 muscles — musculi urethrales — running within the fibrous urethral wall (Figs. 3 m, 4 i, 

 Sd). 



The form of the prostate is one of the most remarkable features of the male urino- 

 genital system of the Cetacea and constitutes one of the chief reasons for regarding 

 the system as extremely primitive in organization. Weber (1904) gives a comparative 

 account of the arrangement of the prostate glands among the Mammalia. In the 

 most primitive forms there is no prostate gland in the strict sense, but an assemblage of 

 racemose urethral glands opening diffusely along the length of the proximal part of the 

 urino-genital canal. These are covered by a layer of striated urethral muscles. In 

 Monotremes these glands have a ring-like arrangement in the neighbourhood of the 

 vasa deferentia. In Marsupials the glandulae urethrales surround the whole length of 

 this part of the urethra (pars pelvica) and form a thick layer, imparting a spindle shape 

 to this part of the canal, which is known as the " prostata ". In Perameles and Phascol- 

 arctos the glandulae urethrales extend dorsally, leaving the distal part of the pars pelvica 

 free, surrounded only by the striated musculus urethralis, which is derived from the 

 sphincter cloacae and thus belongs to the skeletal musculature. In the Cetacea this 

 muscle surrounds the mantle of the urethral glands themselves and thus becomes a 

 compressor prostatae. The arrangement of the glands in the Cetacea is much as in 

 Marsupials, with layers of smooth muscle among the glands themselves under the 

 compressor prostatae. In higher mammals the glands occur outside the smooth muscles, 

 the fibred of which penetrate between the glandular acini, and outside the musculi 

 urethrales, so as to form glandular bodies with separate openings on the colliculus 

 seminalis or its neighbourhood. In these, according to Weber, it is possible to speak of a 

 true prostate: as in the horse where two glandular bodies exist, joined by an isthmus, 

 or in the Primates where the prostate has the form of a single compact mass. Thus the 

 glands of the Monotremes, Marsupials and Cetacea must be said to represent the 

 primitive condition in which the canalis urogenitalis is surrounded by diffuse urethral 

 glands unlocalized to form a prostate. 



The veriimontanam 



At the extreme anterior end of the prostate the dorsal ridge forms a collar-like 

 swelling — the colliculus seminalis or verumontanum (Figs. 3 n, 6) — upon which the 

 urinary canal opens by a small pore (Figs. 3 q, 6 c) and the vasa deferentia by a pair of 

 small apertures immediately behind the urinary opening (Figs. 3 n, 5 a, 6 d). At the 

 verumontanum the crescent-shaped prostatic part of the canalis urogenitalis terminates, 

 ending in two antero-dorsally directed blind angles one on each side of the swollen 

 colliculus (Fig. 6 b). 



The bladder will receive further consideration when the urinary system is described. 

 Its cavity narrows to a thick-walled urinary canal which opens by the small pore above 



