THE CIRCULATION OF THE BLOOD AND LYMPH 159 



(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. 117). 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 

 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 



FIG. 67. PLETHYSMOGRAMS : 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 stimu- 

 lation of the sciatic (i per second) ; on the right hand the effect of rapid stimula- 

 tion (64 per second). In the first case the limb swelled owing to excitation of 

 the vaso-dilators ; in the second, it shrank through excitation of the vaso-con- 

 strictors. 



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 67). 



The Chief Vaso-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 dilatation -of the 

 vessels. This experiment can be very readily performed on the 

 rabbit, and the changes are most easily followed in an albino. 

 The ear on the side of the cut nerve is redder and hotter than 

 the other ; the main arteries and veins are swollen with blood, 



