I 5 c A MANUAL OF PHYSIOLOGY 



corresponds to increase of vascular resistance in it ; increase in the 

 rate of flow implies diminished vascular resistance. Sometimes the 

 red colour of the blood issuing from a cut vein, and the visible pulse 

 in the stream, indicate with certainty that the vessels of the organ 

 have been dilated. 



(5) Alterations in the volume of an organ or limb are often taken 

 as indications of changes in the calibre of the small vessels in it. 

 We have already seen how these alterations are recorded by means 

 of a plethysmograph (p. 116). The brain is enclosed in the skull as 

 in a natural plethysmograph, and changes in its volume may be 

 registered by connecting a recording apparatus with a trephine hole. 



(6) For the separation of the effects of stimulation of vaso- 

 constrictor and vaso-dilator fibres when they are mingled together, as 

 is the case in many nerves, advantage is taken of certain differences 

 between them. For example, the vaso-constrictors lose their excita- 

 bility sooner than the vaso-dilators when cut off from the nerve-cells 

 to which they belong. So that if a nerve is divided, and some days 



FIG. 53. PLBTIVYSMOGRAMS (HIND LIMB OF CAT). (AFTER BOWDITCH 

 AND WARREN.) 



To be read from right to left. On the left hand is shown the effect of slow stimula- 

 tion of the sciatic (i per second) ; on the right hand the effect of rapid stimulation 

 (64 per second). 



allowed to elapse before stimulation, only the dilators will be excited. 

 The vaso-dilators are more sensitive to weak stimuli repeated at long 

 intervals than to strong and frequent stimuli, and the opposite is true 

 of the constrictors. When a nerve containing both kinds of fibres is 

 heated, the excitability of the vaso-constrictors is increased in a 

 greater degree than that of the dilators ; when the nerve is cooled, 

 the dilators preserve their excitability at a temperature at which the 

 constrictors have ceased to respond to stimulation (Fig. 53). 



The Chief Vase-motor Nerves. The first discovery of vaso- 

 motor nerves was made in the cervical sympathetic. When 

 this nerve is cut, the corresponding side of the head, and 

 especially the ear, become greatly injected owing to the 



