THE CIRCULATION OF THE BLOOD. 231 



returned from the plexuses by comparatively small veins; those from 

 the glans and the fore part of the urethra empty themselves into the 

 dorsal veins of the penis; those from the cavernosum pass into deeper 

 veins which issue from the corpora cavernosa at the crura penis; and 

 those from the rest of the urethra and bulb pass more directly into the 

 plexus of the veins about the prostate. For all these veins one condi- 

 tion is the same; namely, that they are liable to. the pressure of muscles 

 when they leave the penis. The muscles chiefly concerned in this action 

 are the erector penis and accelerator urinse. Erection results from the 

 distention of the venous plexuses with blood. The principal exciting 

 cause in the erection of the penis is nervous irritation, originating in 

 the part itself, or derived from the brain and spinal cord. The nervous 

 Influence is communicated to the penis by the pudic nerves, which ram- 

 ify in its vascular .tissue; and after their division in the horse the penis 

 is no longer capable of erection. 



This influx of the blood is the first condition necessary for erection, 

 and through it alone much enlargement and turgescence of the penis 

 may ensue. But the erection is probably not complete, nor maintained 

 for any time except when, together with this influx, the muscles already 

 mentioned contract, and, by compressing the veins, stop the efflux of 

 blood, or prevent it from being as great as the influx. 



It appears to be only the most perfect kind of erection that needs 

 the help of muscles to compress the veins; and none such can materially 

 assist the erection of the nipples, or that amount of turgescence, just 

 falling short of erection, of which the spleen and many other parts are 

 capable. For such turgescence nothing more seems necessary than a 

 large plexiform arrangement of the veins, and such arteries as may ad- 

 mit, upon occasion, augmented quantities of blood. 



The circulation in the Lungs, Liver, and Kidneys will be described 

 under their respective heads. 



The Regulation of the Blood-Flow. 



The flow of blood is not always the same, but varies: 



(a.) With alterations in the force and frequency of the contractions 



of the heart; and 

 (b.) With variations of the peripheral resistance. 



It is obvious that the flow of blood may be increased under the 

 following circumstances: 



(a,) If the force and frequency of the heart's beats be increased, and 

 the peripheral resistance be (1) unchanged, or be (2) diminished. 



(b.) If the force and frequency of the heart be unchanged, and the 

 peripheral resistance be diminished. 



