548 CIRCULATION OF BLOOD AND LYMPH. 



during contraction. Gaskell and others have given reasons for 

 believing that the vessels in the muscles are supplied with vaso- 

 dilator nerve fibers, and Kleen* has shown that mechanical stimula- 

 tion of the muscles kneading, massage, etc. causes a fall of arte- 

 rial pressure. 



Vasodilatation Due to Antidromic Impulses. The existence of definite effer- 

 ent vasodilator fibers in the nerve trunks to the limbs has been made doubt- 

 ful by the work of Bayliss. This author has discovered certain facts which at 

 present tend to make the question of vasodilatation more obscure, but which, 

 when fully understood, will doubtless give us a much deeper insight into the 

 subject. Briefly stated, he has shownf that stimulation of the posterior roots 

 of the nerves supplying the lumbo-sacral and the brachial plexus causes vas- 

 cular dilatation in the corresponding limbs. He has shown that the fibers 

 involved are sensory fibers from the limbs and that therefore when stimulated 

 they must conduct the impulses in a direction opposite to the normal, anti- 

 dromic. It is most difficult to understand how such impulses, conveyed to the 

 terminations of the sensory fibers, can affect the muscular tissue of the blood- 

 vessels. It is most difficult to understand also how such anatomically afferent 

 fibers can be stimulated reflexly in the central nervous system. Bayliss gives 

 reasons for believing that the limbs receive no vasodilator fibers via the sym- 

 pathetic system, and that either the blood-vessels in this region are lacking 

 altogether in such fibers or else the sensory fibers function in the way described. 



General Schema. The main facts regarding the vasomotor 

 apparatus may be summarized briefly in tabular form as follows: 



I. Vasoconstrictor fibers distributed mainly to 

 the skin and the abdominal viscera (splanch- 

 nic area) , all connected with a general center 

 in the medulla oblongata, and in constant 

 tonic activity, 

 nerve libers. } n Vasodilator fibers distributed especially to 



the erectile tissue, glands, bucco-facial region, 

 and muscles; not connected with a general 

 center and not in tonic activity. 



I. Pressor fibers. Cause vascular constriction and 

 rise of arterial pressure from reflex stimula- 

 tion of the vasoconstrictor center e. g., 

 sensory nerves of skin. 

 II. Depressor fibers. Cause vascular dilatation and 

 . ~, , ,, . fall of arterial pressure from reflex inhibition 



,f Q r ^^flfl g o g < of the tonic activity of the vasoconstrictor 



center, e. g., depressor nerve of heart. 

 III. Depressor (or reflex vasodilator) fibers. Cause 

 vascular dilatation and fall of arterial pres- 

 sure from stimulation of the vasodilator 

 center, e. g., erectile tissue, congestion of 

 glands in functional activity. 



It may be supposed that under normal conditions the activity 

 of this mechanism is adjusted so as to control the blood-flow through 

 the different organs in proportion to their needs. When the blood- 

 vessels of a given organ are constricted the flow through that organ 



* Kleen, " Skandinavisches Archiv f . Physiologie," 247, 1887. 



f Bayliss, " Journal of Physiology, " 26, 173, 1900, and 28, 276, 1902. 



Efferent vasomotor 



vasomotor reflexes. 



