150 LECTURE VI. 



are usually grouped together under the title of active 

 hyperaemia, there can be no doubt but that the mus- 

 cular tissue of the arteries is generally essentially con- 

 cerned therein. We very commonly find we have to 

 deal with processes in which the muscular fibres of the 

 vessels have really been stimulated, and the contraction 

 is succeeded by a state of relaxation, such as scarcely 

 ever occurs in an equally marked manner in the rest of 

 the muscles a state which is manifestly the expression 

 of a kind of fatigue and exhaustion, and is the longer 

 persistent, the more energetic the stimulus which was 

 applied. In small vessels with few muscular fibres, 

 therefore, it often seems as if the stimuli really induced 

 no contraction, in consequence of the extreme rapidity 

 with which a state of relaxation is seen to set in, con- 

 tinuing for a considerable time, and allowing of an in- 

 creased influx of blood. 



This same condition of relaxation we can experiment- 

 ally most easily produce by cutting the nerves supplying 

 the vessels of a part, whilst the contraction can be 

 effected to a very great extent by submitting these 

 nerves to a very energetic stimulus. That our acquaint- 

 ance with this kind of contraction is of so late a date, is 

 explained by the fact that the stimuli applied to the 

 nerves must be very powerful, and that, as Claude Ber- 

 nard has shown, only strong electric currents are suffi- 

 cient for the purpose. On the other hand, the condi- 

 tions which ensue upon the section of the nerves are in 

 most parts so complicated, that the dilatation escaped 

 observation, until the lucky spot was discovered also by 

 Bernard, and by the section of the sympathetic nerves 

 in the neck a reliable and convenient field for observa- 

 tion was thrown open to experiment. 



We obtain therefore the important fact that, whether 

 the widening of the vessel, or, in other words, the 



