90 THE INVOL UNTA R Y NER VO US S YS TEM 



must account for such relaxation of the muscular coat of the 

 small arteries. The difficulty of coming to any conclusion upon 

 the cause of the dilatation of the blood vessels in muscles is due 

 to the different effects of large doses of curare in the case of 

 mammals and frogs ; in the first case a large dose of curare will 

 remove both the contraction of the muscle and the dilatation of 

 its blood vessels upon stimulation of the nerve ; in the second, 

 however, even though much curare is given, the vessel may still 

 dilate when the nerve is stimulated. 



In one respect this dilatation in the mylohyoid is peculiar ; 

 it is always very temporary in nature, and in my paper I have 

 given instances where, with a long stimulation of the nerve, the 

 size of the artery measured increased to its maximum, and was 

 back again to the size previous to the stimulation before the 

 stimulation was finished. Also, without any stimulation at all, 

 the calibre of the artery was varying in size considerably, thus 

 producing, as shown in many of my figures, a series of irregular 

 rhythmical dilatations. 



Further, these assumed vaso-dilator fibres to muscle, are al- 

 ways found to be in the same course as the motor nerves to the 

 muscle. Thus although section of the abdominal sympathetic 

 will cause a great increase of flow of blood through the quadri- 

 ceps extensor group of muscles, owing to the removal of tonic 

 constrictor influences, the anterior nerve roots themselves must 

 be stimulated to obtain the great increase of flow accompanying 

 contraction of the muscle. 



We come then to the conclusion that the increased flow of 

 blood through muscle upon stimulation of the motor nerves to 

 that muscle is chiefly due to the direct action upon the small 

 blood vessels of acid metabolites formed by the activity of the 

 muscle, but at the same time the presence of special vaso-dilator 

 nerves is possible. 



In close connexion with the regulation of the vascular supply 

 to muscles is that to glands, and here again the evidence on the 

 whole is in favour of a dilatation due to the metabolites of glan- 

 dular activity rather than to special inhibitory nerve fibres. The 

 problem is of much the same character as that already discussed 

 in the case of muscle, the difference being that in this case it is 

 the action of atropine instead of curare which has given rise to 

 discussion. Keuchel observed that stimulation of the chorda tym- 



