200 VASO-MOTOR NERVES. [BOOK i. 



from the muscle, indicating that the muscular arteries have 

 become dilated; and in the frog this dilation, consequent on 

 section of the nerve, may be actually observed by placing a thin 

 muscle such as the mylo-hyoid under the microscope, and watching 

 the calibre of the small arteries and the circulation of the blood 

 through them while the nerve is being cut. 



We find in fact that in almost all parts of the body certain 

 ' vascular areas ' stand in such a relation to certain nerves that the 

 division of one of these nerves causes a dilation of the minute 

 arteries in, and consequently an increased supply of blood to, a 

 corresponding vascular area. We may speak of these nerves as 

 'vaso-motor' nerves, or more correctly, since in the vast majority 

 of cases the nerves in question have other functions than that of 

 governing arteries, as containing vaso-motor fibres, much in the 

 same way as an ordinary spinal nerve is spoken of as containing 

 sensory and motor fibres ; and from what has been said above it is 

 evident that these vaso-motor fibres are found sometimes in 

 sympathetic, sometimes in cerebro-spinal nerves. 



Since division of a vaso-motor nerve, or nerve containing vaso- 

 motor fibres, leads to the dilation of the arteries of its appropriate 

 vascular area, it is obvious that previous to that division these arte- 

 ries were in a state of permanent constriction, due to a permanent 

 contraction of their muscular coats. This permanent constriction, 

 which may vary considerably in degree (the dilating effects of 

 section of the vaso-motor nerve correspondingly varying in a- 

 mount), is spoken of as ' tone/ ' arterial tone.' Arteries in such a 

 state of permanent constriction as under ordinary circumstances is 

 normal to arteries whose vaso-motor fibres have not been divided 

 and which are otherwise in a normal condition, are said to ' possess 

 tone.' When, as after division of the vaso-motor fibres, the constric- 

 tion gives place to dilation the arteries are said to have 'lost tone;' 

 and when, under various circumstances which we shall study 

 hereafter, the constriction becomes greater than normal, their tone 

 is said to be increased. 



A very little consideration will shew that this arterial tone is 

 a most important factor in the circulation. In the first place the 

 whole flow of blood in the body is adapted to and governed 

 by what we may call the general tone of the arteries of the body at 

 large. In a normal condition of the body, if not all, at least 

 the great majority of the minute arteries of the body are in a 

 state of tonic, i.e. of moderate, constriction, and it is the narrowing 

 due to this constriction which forms a large item of that peripheral 

 resistance which we have seen (p. 129) to be one of the two great 

 factors of blood-pressure. The normal general blood-pressure, and 

 therefore the normal flow of blood, is in fact dependent on the 

 'general tone' of the minute arteries. In the second place, changes 

 in local tone, i.e. the tone of any particular vascular area, have very 

 decided effects on the circulation. These effects are both local 

 and general, as the following considerations will shew. 



