154 THE CIRCULATION. 



influence of the nerves ; but the mode in which they excite a 

 greater influx of blood is not with certainty known. 



The most probable explanation is that offered by Professor 

 Kolliker, who ascribes the distension of the venous plexuses to 

 the influence of organic muscular fibres, which are found in 

 abundance in the corpora cavernosa of the penis, from the 

 bulb to the glans, also in the clitoris and other parts capable 

 of erection. While erectile organs are flaccid and at rest, 

 these contractile fibres exercise an amount of pressure on the 

 plexuses of vessels distributed amongst them, sufficient to pre- 

 vent their distension with blood. But when through the in- 

 fluence of their nerves, these parts are stimulated to erection, 

 the action of these fibres is suspended, and the plexuses thus 

 liberated from pressure, yield to the distending force of the 

 blood, which, probably, at the same time arrives in greater 

 quantity, owing to a simultaneous dilatation of the arteries of 

 the parts, and thus the plexuses become filled, and remain so 

 until the stimulus to erection subsides, when the organic mus- 

 cular fibres again contract, and so gradually expel the excess 

 of blood from the previously distended vessels. The influence 

 of cold in producing extreme contraction and shrinking of 

 erectile organs, and the opposite effect of warmth in inducing 

 fulness and distension of these parts, are among the arguments 

 used by Kolliker in support of this opinion. 



The accurate dissections and experiments of Kobelt, extend- 

 ing and confirming those of Le Gros Clark and Krause, have 

 shown, that this influx of the blood, however explained, is 

 the first condition necessary for erection, and that through 

 it alone much enlargement and turgescence of the penis may 

 ensue. But the erection is probably not complete, nor main- 

 tained 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 tur- 

 gescence nothing more seems necessary than a large plexiform 

 arrangement of the veins, and such arteries as may admit, 

 upon local occasions, augmented quantities of blood. 



The Influence of the Nervous System on the circulation in 

 the bloodvessels will be considered in Chapter XVII. 



