VASO- MOTOR NERVES AND NERVE-CENTRES. 279 



Vasodilator Nerves. We have already noticed, in the 

 case of the stomach, one method by which a locally increased 

 blood-supply may be brought about in an organ while it is 

 at work, viz., by inhibition of local vaso- constrictor fibres. 

 Frequently, however, in the Body this is managed in 

 another way; by efferent vaso-dilator nerves which inhibit 

 or paralyze, not the vaso-constrictor centre, but the muscles 

 of the blood-vessels directly. The nerves of the skeletal 

 muscles for example contain two sets of efferent fibres: one 

 motor proper and the other vaso-dilator. When the muscle 

 contracts in a reflex action or under the influence of the 

 will both sets of fibres are excited ; so that when the organ is 

 set at work its arteries are simultaneously dilated and more 

 blood flows through it. But if the animal have previously 

 administered to it such a dose of curare as to just paralyze 

 the true motor-fibres, stimulation of the nerve produces 

 dilatation of the arteries without a corresponding muscular 

 contraction. Quite a similar thing occurs in the salivary 

 glands. Their cells, which form the saliva, are aroused to 

 activity by special nerve-fibres; but the gland-nerve also 

 contains a quite distinct set of vaso-dilator fibres which nor- 

 mally cause a simultaneous dilatation of the gland-artery, 

 though either can be artificially stimulated by itself and 

 produce its effect alone. Through such arrangements the 

 distribution of the blood in the Body at any moment is gov- 

 erned : so that working parts shall have abundance and other 

 parts less, while at the same time the general arterial pressure 

 remains the same on the average; since the expansion of a 

 few small local branches but little influences the total periph- 

 eral resistance in the vascular system. Moreover, commonly 

 when one set of organs is at work with its vessels dilated, 

 others are at rest with their arteries comparatively contracted, 

 and so a general average blood -pressure is maintained. Few 

 persons, for example, feel inclined to do brain-work after a 

 heavy meal: for then a great part of the blood of the whole 

 Body is led off into the dilated vessels of the digestive organs, 

 and the brain gets a smaller supply. On the other hand, when 

 the brain is at work its vessels are dilated and often the whole 

 head flushed : and so excitement or hard thought after a meal 

 is very apt to produce an attack of indigestion, by diverting 

 the blood from the abdominal organs, where it ought to be at 

 that time. Young persons, whose organs have a superabuu- 



