352 EXAMPLES OF TASO-MOTOR ACTIONS. [BOOK i. 



may be brought about. When the vascular area so affected is 

 small the effects are local, more or less blood is distributed through 

 the area ; when the vascular area affected is large, the inhibition 

 of constriction may lead to a marked fall, and an augmentation of 

 constriction to a marked rise of general blood pressure. Broadly 

 speaking, we may say that whenever a vascular change is needed 

 for the general well-being of the economy, it is this vaso-constrictor 

 system which is called into play. 



The distribution of clearly proved vaso-dilator fibres is as we 

 have said very limited, and even the vaso-constrictor fibres are 

 most abundant in the nerves going to the skin and to the viscera. 

 In respect to the arteries supplying the numerous skeletal muscles, 

 there is much dispute as to whether they are supplied by vaso- 

 dilator fibres ; and the supply of vaso-constrictor fibres to them 

 is at least not large. We may perhaps infer that the vascular 

 changes in the muscles are intended chiefly for the benefit of 

 the muscles themselves, and are not to any great extent, like those 

 of the skin and viscera, utilized for the more general purposes 

 of the economy. 



179. We shall have occasion later on again and again to 

 point out instances of the effects of vaso-motor action both local 

 and general, but we may here quote one or two characteristic 

 examples. "Blushing" is one. Nervous impulses started in some 

 parts of the brain by an emotion produce a powerful inhibition of 

 that part of the bulbar vaso-motor centre which governs the 

 vascular areas of the head supplied by the cervical sympathetic, 

 and hence has an effect on the vaso-motor fibres of the cervical 

 sympathetic almost exactly the same as that produced by section 

 of the nerve. In consequence the muscular walls of the arteries of 

 the head and face relax, the arteries dilate and the whole region 

 becomes suffused. Sometimes an emotion gives rise not to blushing, 

 but to the opposite effect, viz. to pallor of the face. In a great 

 number of cases this has quite a different cause, being due to a 

 sudden diminution or even temporary arrest of the heart's beats ; 

 but in some cases it may occur without any change in the beat of 

 the heart, and is then due to a condition the very converse of that 

 of blushing, that is, to an increased arterial constriction ; and this 

 increased constriction, like the dilation of blushing, is effected 

 through the agency of the central nervous system and the cervical 

 sympathetic. Blushing and its opposite pallor are most marked in 

 the face ; but other parts of the body may blush (or grow pale) 

 the change being brought about by appropriate nerves. 



The vascular condition of the skin at large affords another 

 instance. When the temperature of the air is low the vessels of the 

 skin are constricted, and the skin is pale ; when the temperature of 

 the air is high the vessels of the skin are dilated, and the skin is 

 red and flushed. In both these cases the effect is mainly a reflex one, 

 it being the central nervous system which brings about augmen- 



