350 PHYSIOLOGY CHAP. 



phenomenon, viz. fall of arterial and rise of venous pressure, 

 greater than that which occurs on simple division of the nerve 

 (local circulatory delay) in consequence of fatigue (Fig. 159). 



The fact of vascular ultra-dilatation is very interesting, as it 

 implies that strong and persistent excitation of the nerve exhausts 

 the peripheral ganglia, and thus depresses the tone which these 

 maintain in the vessels even after the nerve has been divided. 



IV. Since 1874, the theory of vaso- dilator nerves has been 

 much developed. Next to Bernard's discovery of the vaso- 

 dilator fibres contained in the chorda tyrnpani (referred to 

 above), the discovery of the nervi erigentes of the penis by 

 Eckhardt (1863) demands special mention. These nerves are 



FIG. 159. Effect on arterial and venous pressure in facial vessels of electrically exciting the 

 peripheral trunk of horse's cervical sympathetic. (Dastre and Morat.) Af, tracing of blood 

 pressure in peripheral trunk of facial artery ; Vf, of facial vein. The excitation took place at 

 the part between the two vertical lines, marked on abscissa by an electric signal separate from 

 that which shows the time in seconds. The tracings show that stimulation of the sympathetic 

 is followed by a rise of arterial and fall of venous pressure (preceded by temporary rise due 

 to increased outflow), which is succeeded by the opposite effect, i.e. fall of arterial, and rise 

 of venous pressure, due to extra-dilatation of vessels. 



branches from the sacral plexus, which, when peripherally stimu- 

 lated, cause erectile swelling of the corpora cavernosa, due not 

 to obstruction of the venous outflow, but to increased arterial 

 influx, owing to active dilatation of the helicine arteries. These 

 two discoveries, however, remained isolated for more than a decade, 

 and Goltz (1874) was the first to suggest that the dilators, like 

 the constrictors, were distributed to every vascular region, the 

 difficulty of experimental proof arising from the fact that they 

 nearly always run jointly with the constrictor fibres, which by 

 their prevailing influence on the tone of the vessels mask the 

 antagonistic action of the dilators. 



For this reason it cannot be decided whether the dilator nerves 

 are, like the constrictors, in tonic activity ; section of the chorda 

 tympani or nervi erigentes produced no perceptible constriction of 

 the vessels to which they are distributed. 



