CH. XXII.] VASO-DILATOR NERVES 303 



The new fibres which arise in the ganglia are usually non-medul- 

 lated, and are termed post-yanglionic. Those for the body wall and 

 limbs pass back from the sympathetic ganglia to the spinal nerves 

 by the grey rami communicantes, and are distributed with the other 

 spinal nerve-fibres. The cell-stations for the upper limb fibres are 

 in the ganglion stellatum, and for the lower limb fibres in the lower 

 lumbar and upper sacral ganglia. 



Those for the interior of the body pass into the various plexuses 

 of sympathetic nerves in the thorax and abdomen, and are distributed 

 to the vessels of the thoracic and abdominal viscera. This set includes 

 the most important vaso-motor nerves of the body, the splanchnics. 

 Their cell-stations are situated in the various collateral ganglia. 



The vaso-dilator nerves have been stated to accompany those 

 just described, but they are not limited to the outflow from the 

 second thoracic to the second lumbar. Thus, the nervi erigentes 

 originate as white rami communicantes from the second and third 

 sacral nerves, and the chorda tympani, another good example of a 

 vaso-dilator nerve, is a branch of the seventh cranial nerve. 



Our knowledge of vaso-dilator nerves is limited, except in such 

 instances as the two nerves just mentioned. Equally deficient is our 

 information concerning vaso-dilator centres in the central nervous 

 system. W. M. Bayliss, in his search for vaso-dilator fibres in the 

 dog, was not successful in finding any for the hind limb in the 

 abdominal sympathetic chain; but the only fibres, excitation of 

 which produced vascular dilatation there, are contained in the 

 posterior roots. He also found fibres in the posterior roots of the 

 12th and 13th thoracic nerves, which act as vaso-dilators of the 

 small intestine. Not only is vaso-dilatation the result of mechanical, 

 or electrical stimulation of these roots, but experiments are adduced 

 which show that in normal reflexes, such as occur when the 

 depressor nerve is stimulated, the dilator impulses travel by the 

 same route. This raises the question whether the posterior roots 

 contain true efferent fibres. The facts of degeneration show that 

 they do not. Bayliss is therefore driven to the conclusion that the 

 same nerve terminations in the periphery serve both to take up 

 sensory impressions and to convey inhibitory impulses to the 

 muscular structures in which they end. In other words, we have 

 here another example which may be added to those previously 

 mentioned (p. 160), that nerve-fibres may convey impulses in both 

 directions. The term antidromic is used by Bayliss to express the 

 fact that impulses may travel in the reverse direction to that in 

 which they usually pass. 



The Vaso-motor centre can be excited directly by induc- 

 tion currents; the result is an increase of arterial blood-pressure 

 owing to an increase of the contraction of the peripheral arterioles. 



