THE MALE AND FEMALE REPRODUCTIVE ORGANS 1127 



erection of the penis, the male organ of copulation. It is composed 

 chiefly of cavernous tissue which is arranged in three long and some- 

 what cylindrical masses, forming the corpus spongiosum below and 

 the two corpora cavernosa, one on each side, above. The former is 

 traversed by the urethra and terminates anteriorly in a conical struc- 

 ture, the glans penis. Externally, these bodies are enveloped by 

 fibrous sheaths and a thin layer of very movable and distensible skin, 

 which is then reflected upon the glans penis to form the prepuce. 

 The erectile tissue of which these bodies are composed, is made up of 

 cavernous spaces which are really venous sinuses lined with a layer 

 of flattened epithelium. Their walls consist of membranous parti- 

 tions which are derived from the external fibrous investment, the tunica 

 albuginea, as well as from the median septum of the penis. In this 

 way ^ a spongy network of connective tissue is formed which is much 

 denser near the circumference than near the center. The central 

 spaces, therefore, are larger than the outer ones, but all of them are 

 supplied with blood from branches of the internal pudendal artery 

 through capillaries which are rather more widely open than those of 

 other tissues. 



The erection of this tissue is dependent upon a dilatation of these 

 afferent channels through which the blood is then poured freely into 

 the lacunae, but since the venous collecting tubules begin with a nar- 

 row orifice which is strengthened by rings of smooth muscle tissue, 

 th3 oflflow is somewhat hindered both in a mechanical way as well as by 

 an active constriction of these sphincters. Consequently, the erection 

 of the penis cannot be regarded as a pure vaso-dilator phenomenon, 

 but as an active venous retardation which is brought about chiefly by 

 the contraction of those muscle cells with which the outlets of the 

 several blood-spaces are beset. In addition, it is entirely probable that 

 the action of this intrinsic muscle tissue is materially strengthened by 

 the contraction of certain extrinsic muscles, such as the ischio and 

 bulbo-cavernosi. By compressing the larger collecting channels, 

 these muscles tend to raise the venous pressure without actually 

 blocking the return of the blood. Meanwhile, the inner walls ofthe 

 cavernous spaces are fully exposed to the arterial blood-pressure,^ which 

 causes them to move outward as far as their tough fibrous constituents 

 as well as the fibrous investment of the entire organ will allow. 



The length of time during which copulation must be continued in 

 order to give rise to an ejaculation of the semen differs greatly with 

 the condition and type of the animal. It is safe to assume, however, 

 that the erection of the penis cannot be attained by vaso-dilatation 

 alone, because a reaction of this kind is neither sufficiently intense nor 

 lasting. Nor can it be due to venous stagnation alone, because the 

 erected organ does not become cyanosed and retains a higher tempera- 

 ture throughout this act. These facts unmistakably point toward a 

 more copious blood supply and greater through-flow and not merely 

 1 Frangois-Frank, Arch, de Physiol., 1895, 122. 



