Active Hypercemia. 



385 



normal functional activity of a particular organ, reflex excitation of the vaso- 

 constrictor nerves of the resting organs liherates a maximum blood supply for 

 the working organ ; this is a regulating mechanism of the central nervous 

 system which ensures a more efficient functional hyperemia of the working 

 organ (Loven, 1866 ; Bayliss, 1920). 



(ii) Vaso-dilator Nerves (neuro-tonic hyperemia). — The first evidence for 

 the existence of vaso-dilator nerves was supplied by Claude Bernard (1858), 

 who found that stimulation of the chorda tympani caused not only secretion 

 by the submaxillary gland, but also dilatation of its blood vessels. Vulpian 

 showed the reddening of the tongue upon stimulation of the lingual nerve, 

 and proved that the fibres concerned came from the chorda tympani. Goltz 

 had noticed that whereas stimulation of the sciatic nerve, when freshly cut, 

 caused pallor of the skin of the toes and a lowering of temperature as 

 measured by a thermometer placed between the toes, the same stimulation of 

 the nerve two days after section caused reddening and a rise of temperature. 

 He concluded that vaso-dilator, as well as vaso-constrictor nerves, existed in 

 the sciatic, and that they resisted degeneration for a longer time than the 

 vaso-constrictors. Bernstein showed that the vaso-dilator fibres in the sciatic 

 could be excited independently by means of slow rhythmic shocks, especially 

 in a leg which had been previously cooled. 



The remarkable nature of the vaso-dilator supply to the limbs, skin of the 

 trunk, and probably of the ears and face, and to the intestine, was not explained 

 until 1901, when Bayliss investigated vaso-dilation in these regions produced 

 by stimulation of the peripheral cut ends of the corresponding dorsal roots. 

 The work of Head and Campbell (1900), on herpes zoster, shows this disease 

 to be due to irritative lesions of the dorsal root ganglia, which give rise to 

 abnormal impulses in an efferent (anti-dromic) direction along the sensory 

 fibres to the skin ; whether stimulation of vaso-dilator nerves can produce 

 blisters has not yet been proved. Ninian Bruce (1910) showed that the 

 " neurotonic hyperemia " (Adami), produced in the conjunctiva by an irritant 

 like oil of mustard, could be prevented by first paralysing the sensory nerves 

 with cocaine, or by section and degeneration ; this was explained as an " axon 

 reflex," since the hyperemia depends on the integrity of the sensory fibre. 



(iii) Vaso-dilator Substances (a local hyperaemia). — It is generally held that 

 the hyperaemia of inflammation arises from the action of physical and chemical 

 agents directly upon the blood-vessel wall. The great similarity between a 

 wheal produced by the lash of a whip, for example, and the effects of a local 

 introduction of histamine (/3-iminazolyl-ethylamine) into a lightly scarified 

 area of skin lends support to this view, since the depressant action of histamine 

 locally upon the capillary wall is definitely known (Dale and Eichards). 



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