10 



nerves in certain parts of the ))ody which are said to dilate 

 the same when stimulated, and in certain nerves both kinds 

 of libers have been discovered. Several authors state how- 

 ever, that the action of the dilator neives is of an inhibitor^/ 

 character. 



One writes that "inhibition is connected with tlie effect 

 of two sets of impulses upon the responding cell, and the 

 two stimuli must tend to excite different reactions." There 

 is little doubt from a consideration of the subject but that 

 the cause of the relaxation is a removal of the normal tonic 

 action by inhibition of the activity of the constrictor nerve 

 centers. The l^lood vessels themselves would in that case be 

 temporarily without stimulation, and practically in the same 

 condition as when a nerve is paralyzed or when it is severed, 

 as in the examples above stated. Inhibition, which checks 

 or restrains the constrictor nerve centers from sending' out 

 their ordinary impulses, is a totally different thing from 

 stimulating them to action. From the foregoing facts it 

 would seem therefore, that while the relaxation is due to the 

 action of the nerves, the action is a negative one, and that 

 no stiviidation is transmitted from the central ner\'ous system 

 to the coat of the blood vessels I 



But, when the nerves resunie their activity and stimulate 

 the muscular fibers, those libers again contract, and the 

 result is, smaller tubes, and lessened color I 



It should not ])e overlooked, while referring to the action 

 of nerves, that muscle has in itself, wholly independent of 

 the nerves, the properties of contractility, irritability, and 

 conductivity, and that therefore muscular liber can be ex- 

 cited to action not only indirectly by the nerves, but by 

 application of the stimulus directly to the nmscle. Inas- 

 much as vegetable tissue has precisely these same })roperties 

 it can well be imagined that plants and animals have many 

 functions of a similar nature : indeed many of the lower 

 animals have no nerves at all, and so nearly resemble vege- 

 table tissue that the two classes are with difficulty distin- 



