256 CIRCULATION OF THE BLOOD. 



of seconds. If no increase of arterial pressure takes place, the 

 secondary coil, which in beginning the experiment must be 

 distant from the primary one, is cautiously brought nearer to 

 it until this effect is produced. As soon as this is the case, 

 it is usually observed that the artery of the ear, instead of 

 contracting, dilates, and that the whole lobe obviously con- 

 tains more blood than it did before. Frequently, however, it 

 happens that, notwithstanding the increase of arterial press- 

 ure, no increased vascular injection is observable. In this 

 case, recourse must be had to the posterior auricular nerve, 

 the excitation of the central end of which is almost certain to 

 be followed by the effect in question. The augmentation of 

 arterial pressure and the dilatation of the auricular artery 

 appear to be collateral phenomena, both increasing gradually 

 during the few seconds which succeed the commencement of 

 electrical excitation. If care is taken neither to prolong the 

 excitation unduly nor to use too strong currents, the reaction 

 may be witnessed a great number of times in the same animal. 



54. (2.) Excitation of the Dorsalis Pedis. When the 

 central end of the divided dorsal nerve of the foot is excited, 

 phenomena occur of a similar nature. To enable the observer 

 to judge of the effect, the saphenous artery must be exposed 

 in its course down the inner side of the lower half of the thigh, 

 as recommended in 49. It is then seen that during and 

 after excitation of the central end of the divided nerve, the 

 artery gradually dilates, subsequently regaining its former 

 dimensions. 



The general result of the preceding experiments may be 

 expressed by saying that the afferent nerves to which the} r 

 relate (in common probably with other sensory nerves) con- 

 tain fibres so endowed that, when they are excited, the action 

 of the vasomotor centre is inhibited or suspended, as regards 

 certain regions with which the nerves in question are in close 

 anatomical relation. In its relations to the vasomotor ner- 

 vous system, the words " inhibitory" and "depressor," both 

 of which are used by ph^ysiologists to denote the case in which 

 arterial tonus is diminished by excitation of an afferent nerve, 

 may be regarded as equivalent. 



EXPERIMENTS RELATING TO THE EFFECTS OF DIRECT EXCITATION 

 AND DIVISION OF THE VASOMOTOR NERVES. 



When a vasomotor nerve is excited directly, the arteries of 

 the region to which it is distributed contract. When it is 

 divided, they become permanently larger, and remain unaffect- 

 ed by changes in the condition of the vasomotor centre, 

 whether these are determined by direct or reflex excitation. 



