244 THE PHYSIOLOGY OF REPRODUCTION 



When the venous spaces in the erectile tissue are distended 

 with blood the organ erects, becoming hard and rigid in con- 

 dition. It is this power of erection which enables the penis 

 to function as an intromittent organ during copulation. 



The above description applies more especially to the 

 copulatory organ in Man. In the other groups of Mammals 

 it has essentially the same structure, but presents sundry 

 modifications in the different orders. 1 In the Monotremata, 



Fio. 59. Section through erectile tissue. (After Cadiat, from Schafer.) 

 a, trabeculse ; b, venous spaces ; c, muscular fibres cut across. 



however, there is no corpus spongiosum. 2 In some Mammals 

 (Carnivora and Rodentia) the penis is provided with a 



1 For an account of the structure of the copulatory organ in the various 

 groups of Vertebrates, with notes on the different modes of copulation and 

 bibliography, see Gerhardt, " Morphologische und biologisfihe Studien iiber 

 die Kopulationsorgane der Saugethiere," Jenaische Zeitschr. f. Naturwissen- 

 schaft, vol. xxxix., 1905. 



2 The penis of the Monotreme is perforated by a canal, through which the 

 semen passes but not the urine. When in a relaxed state the organ lies in a 

 little pouch in the floor of the cloaca, from which it projects when erected. 

 The cloaca is the single common chamber through which the faeces and 

 urine pass to the exterior, as in birds and reptiles. In birds the penis is 

 either altogether absent or else is "rudimentary (Crax, Cryptums, Lamel- 

 lirostres, Ratitae), Disselhorst, " Gewichts- und Volumszunahmedermannlichen 

 Keimdriisen," &c., Zoo/. Am., vol. xxxii., 1908. 



