ERECTION. 955 



The wall is thicker and the color is deeper, so that the body at the time of labor 

 still measures from 6 to 10 mm. in diameter and its remains may be recognizable 

 even after the lapse of years. The yellow body after pregnancy is designated 

 the true corpus luteum (Fig. 367). 



ERECTION. 



The knowledge of the distribution of the blood in the penis is due to the in- 

 vestigations of C. Langer. The albuginea of the cavernous bodies consists of 

 tendinous connective tissue, closely reticulated elastic tissue and unstriated 

 muscle-fibers, which form a firm fibrous envelop, from which innumerable trabec- 

 ulas of similar structure pass inward, so that the cavernous bodies acquire the con- 

 figuration of a sponge. The anastomosing spaces thus produced form a labyrinth 

 of venous sinuses, which are lined by endothelium. The largest of these spaces 

 are situated in the lower, outer portion of the cavernous body; in the upper 

 portion the spaces diminish in number and size. The smaller arteries of the 

 cavernous bodies arise from a branch of the arteria profunda of the penis running 

 along the septum and they reach the trabeculae in a tortuous course. Some of 

 the small arterial branches in the cortical areas pass directly over into the larger 

 venous sinuses; but similar direct transition from arteries to venous spaces takes 

 place also in the interior of the cavernous bodies. A capillary network occurs 

 also in the cortex and in the interior of the cavernous bodies /opening into the 

 venous spaces. The helicine arteries of the penis described by Johannes Miiller 

 are only more or less incompletely injected arterial loops bent upon themselves, 

 whose occurrence is due to the cord-like course of the trabeculae. From the in- 

 terior of the cavernous bodies, the venas profundse of the penis arise by means 

 of fine branches. In addition venous branches pass from the cavernous spaces 

 to the dorsum of the penis, uniting to form the dorsal vein of the penis. As 

 these branches pass through the meshes of the vascular network in the cortex 

 of the cavernous bodies, it is obvious that constriction of the meshes resulting 

 from congestion of the network must cause compression of the efferent branches. 



The spongy body of the urethra consists for the greater part of an outer layer 

 of anastomosing veins lying close together, surrounding the longitudinal vessels 

 of the urethra. 



In the dog all of the arteries of the penis pass toward the surface, where they 

 divide in tuft-like fashion. The veins arise from the capillary loops of the papillae 

 and they convey their blood into the cavernous bodies. Onlv a small amount of 

 blood reaches the cavernous spaces through internal capillaries and veins; and 

 arterial blood never flows directly into them. 



The mechanism of erection consists in a marked distention of the 

 blood-vessels of the penis, with fourfold or fivefold increase in volume, 

 elevation of temperature, increase of blood-pressure within its vessels 

 to one-sixth of the carotid pressure and initial pulsatory movement, 

 increased consistency and erection, with a direction of the organ 

 in conformity with the curvature of the vagina. The preliminary 

 process consists in a marked increase in the arterial supply of blood, 

 the arteries becoming dilated and pulsate strongly. This process is 

 controlled by the erector nerves, which arise principally from the second 

 (less commonly from the third) sacral nerve (in the dog) and possess 

 ganglion-cells in their course. These nerves, belonging to the vaso- 

 dilators, may be in part stimulated reflexly by irritation of the sensory 

 nerves of the penis, the transference of the irritation taking place in the 

 erection-center in the spinal cord. Thus, also, sensory irritation induced 

 by voluntary movements of the genitalia, may excite this reflex through 

 the ischiocavernosus and bulbocavernosus and the cremaster muscles, 

 even the conception of sensory irritation of the penis may be attended 

 with the same results. 



Some vasodilators pass (in the dog) also through the lumbar sympathetic 

 and the internal pudendal nerve. The last-named nerve usually contains vaso- 

 constrictor fibers for the penis, although the erector nerves contain some. 



