CH. XXL] VASO-MOTOR NERVES 301 



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. Bayliss has also shown 

 that the posterior root fibres may act as vaso-dilators (see p. 303). 



All vaso-motor nerves, whether they are constrictor or dilator, 

 differ very markedly from the spinal nerve-fibres which are distri- 

 buted to voluntary muscles in being ganglionated ; that is, in having 

 cell stations or positions of relay on their course from the central 

 nervous system to the muscular fibres they supply. 



The existence of cell stations between the central nervous 

 system and the muscular fibres is not confined to the nerves of 

 blood-vessels, but is found also in the nerves which supply the 

 heart and other viscera. 



Moreover, the nerves which supply the voluntary muscles are 

 motor in function ; inhibitory fibres to the voluntary muscles of 

 vertebrates do not exist. But in the case of the involuntary muscles 

 there are usually the two sets of nerve-fibres with opposite 

 functions. 



In the case of the heart, we have an accelerator set which course 

 through the sympathetic, and an inhibitory set which course through 

 the vagus. 



In the case of the vessels, we have an accelerator set, which we 

 have hitherto called vaso-constrictors, and an inhibitory set we have 

 been calling vaso-dilators. 



In the case of the other contractile viscera, we have also viscero- 

 accelerator and viscero-inhibitory, which respectively hasten and 

 lessen their peristaltic movements. 



Adopting Gaskell's nomenclature, we may further term the 

 accelerator groups of nerves katdbolic, as they increase the activity 

 of the muscles they supply, bringing about an increase of wear and 

 tear, and an increase in the discharge of waste material. The 

 inhibitory nerves, on the other hand, are anabolic, as they produce a 

 condition of rest in the tissues they supply, and so give an oppor- 

 tunity for repair or constructive metabolism. 



The distribution of the vaso-motor nerves and the viscero-motor 

 nerves has been within recent years very thoroughly worked out 

 by Langley. The nerves of the various viscera we shall take with 

 the individual organs. In all these cases, there is a cell station 

 somewhere in the sympathetic system, and only one for each nerve- 

 fibre. The preganglionic fibres (i.e., the fibres from the spinal cord to 

 the sympathetic cell station) are usually medullated ; the postgang- 

 lionic fibres (i.e., those that leave the ganglion) are usually non- 

 medullated. But this histological distinction, so much emphasised 

 by Gaskell, is not without exceptions, and the localisation of cell 

 stations is made with far greater certainty by Langley's nicotine 



