256 THE PHYSIOLOGY OF REPRODUCTION 



smaller arteries are stated to communicate directly with the cavernous 

 veins. The blood is carried away by two sets of veins, the one set 

 uniting to form the dorsal vein, and the other communicating with 

 the prostatic plexus and the pudendal veins. 



When the venous spaces in the erectile tissue are distended with 

 blood the organ erects, becoming hard and rigid in condition. . 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 



FIG. 68. Section through erectile tissue. (After Cadiat, from Schafer.) 

 a, Trabeculae ; b, venous spaces ; e, muscular fibres cut across. 



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, however, there is no corpus 

 spongiosum. 2 In some Mammals (Carnivora and Eodentia) the 

 penis is provided with a cartilaginous or bony support, the os penis, 

 which is developed especially in the region of the glans. It is 



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 biologische Studien iiber 

 die Kopulationsorgane der Saugethiere," Jenaische Zeitsch. f. Naturwissenschaft, 

 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, Crypturus, Lamellirostres, 

 Ratitae), Disselhorst, "Gewichts- und Volumszunahme der mannlichen Keim- 

 driisen, etc.," Zool. Anz., vol. xxxii., 1908. 



