INFLUENCE OF THE NERVOUS SYSTEM. 173 



of light produces darkness by joining light to light. Claude 

 Bernard appears to have adopted this hypothesis, and it may 

 also serve to explain the nervous mechanism of the afflux of 

 blood in erection : the nerves coming from the cord act upon 

 the threads of the great sympathetic nerve so as to prevent 

 their action, causing turgescence and hyperaemia of the erectile 

 tissue. Section of the cord does not cause continuous erec- 

 tion, because the nervous influx of the spinal (rachidian) 

 nerves can no longer act upon the sympathetic nerves, and 

 this association of nervous influences is alone capable of pro- 

 ducing vaso-motor paralysis. In adopting this hypothesis, 

 the influence of the first nerve on the second must be con- 

 sidered as equivalent to the section of the great sympathetic 

 nerve made by an operator who desires, for instance, to pro- 

 duce hypercmia of a rabbit's ear. 



This view, however, does not satisfy all who have made 

 the experiment, because some among them have been con- 

 vinced that more serious hyperasmia takes place under the 

 influence of reflex phenomena in a less degree than any 

 which may be caused by section of the great sympathetic 

 nerve in the same parts : the idea has thus been suggested 

 of active hypercemia, more intense than passive or paralytic 

 hypercemia, and two theories have lately been formed on this 

 subject; that of Schiff, or active dilatation of the vessels; 

 that of Legros and Onimus, or peristaltism of the vessels. 



This theory of the active dilatation of the vessels was for 

 a short time entertained by Cl. Bernard, but he now appears 

 to have finally renounced it : it is not easy to prove it by 

 anatomy, for it supposes the existence of longitudinal mus- 

 cular fibres in the coats of the arteries, and of these histology 

 shows no trace. SchifF, therefore, carefully abstains (" Legons 

 sur la Physiologie de la Digestion ") from stating his theory 

 in explicit terms; he still regards as inexplicable both the 

 origin and the mode of action of these dilating nerves, but he 

 relates many experiments, which, to his view, make their exist- 

 ence undeniable. 



He observed, in the small arteries of a rabbit's ear, phe- 

 nomena of systole and diastole, appearing from 2 to 8 times 

 in a minute (this by no means coincides with the beating of 

 the heart). These movements cannot be the consequence 

 of alternate contractions of the veins, for direct inspection of 

 these vessels reveals nothing of the kind ; neither are they 

 caused by paralysis of the arteries succeeding to a momen- 



