I338 HANDBOOK OF PHYSIOLOGY ^ CIRCULATION II 



fig. 9. Abscissae: time in days, the vertical broken line indicating the day on which sympathectomy 

 was performed. Ordinatcs: blood flow in ml/100 ml tissue/min. The averaged results of experiments 

 on five hands and six feet show the effect of sympathectomy on the blood flow in the hand and 

 foot, and on the skin temperatures of the fingers and toes. The denervations were performed to 

 prevent excessive sweating; hence the responses are those of normal human blood vessels. Note the 

 transient increase in blood flow after operation and the subsequent decline as intrinsic tone returns 

 to the vessels. [From Barcroft (18).] 



VASODILATATION CAUSED BY ANTIDROMIC STIMULATION 

 OF DORSAL ROOT SENSORY NERVES: THE AXON REFLEX 



pathway. Stimulation of the peripheral end of a 

 cut sensory nerve often causes vasodilatation in the 

 area of skin supplied by the nerve, and Bayliss (24) 

 showed that this was due to impulses traveling toward 

 the periphery along neurons of the dorsal root system. 

 The vasodilator effect of such antidromic nervous 

 impulses is considerable, but evidence has not been 

 forthcoming for the use of this route from the central 

 nervous system to the periphery for any reflex ad- 

 justments of the circulation. It seems probable that 

 the artificially provoked antidromic impulses travel 

 to the vascular nerve endings of the peripheral axon 

 reflex pathway. The usual source of impulses ar- 

 riving here is from nearby sensory- nerve endings, 

 probably subserving pain sensation. The transmitter 

 substance for the vasodilatation produced in the 

 chronically sympathectomized ear of the rabbit by 

 stimulation of the sensory great auricular nerve is 

 neither histamine nor acetylcholine (123) but is 

 probably adenosine triphosphate (122), the presence 

 of which has been demonstrated in nerve roots (121). 



Innervation of the Blood Vessels of the Skin 

 in Different Areas 



The problem is to define, for different areas of 

 skin, the existence and range of action of vasocon- 

 strictor and vasodilator fibers. This has been most 

 fully investigated in the human limbs, particularly 

 in the upper limbs, and the innervation here will be 

 first described. 



the human hand and fingers. In the cold subject, 

 the blood flow through the hand is very small, often 

 less than 1 ml per 100 ml per min. When the subject 

 is warmed, either in a hot cabinet (143) or by im- 

 mersing the legs in stirred water at 44 C (89), the 

 blood flow increases to about 30 ml per too ml per 

 min, this being part of the general response by which 

 the body attempts to lose heat. Although abundant 

 cholinesterase is found in the arteriovenous anastomo- 

 ses of the fingers, and although Lewis & Pickering 

 (143) obtained evidence in cases of Raynaud's dis- 

 ease for the activity of vasodilator nerves, the increase 

 in blood flow in normal persons can be entirely ac- 

 counted for by a reduction in the activity of vaso- 

 constrictor nerves; several careful investigations have 



