286 EXAMPLES OF VASO-MOTOR ACTIONS. [BOOK L 



essentially local in nature. When any set of the fibres come into 

 action the vascular area which these govern is dilated ; and the 

 vascular areas so governed are relatively so small that changes in 

 them produce little or no effect on the vascular system in general ; 

 the fibres are called into play to produce special effects in special 

 organs. 



The effects of changes in the activity of the vaso-constrictor 

 fibres are both local and general. They are also double in nature ; 

 by an inhibition of tonic constrictor impulses a certain amount of 

 dilation may be effected ; by an augmentation of constrictor im- 

 pulses, constriction, it may be of considerable extent, 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 constric- 

 tion may lead to a marked fall, and an augmentation of constric- 

 tion 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 mus- 

 cles, 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 vascu- 

 lar 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. 



157. 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 inhibi- 

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

 the vascular areas of the head supplied by the cervical sympa- 

 thetic, 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 ; 



