

CH. xxi.] VASO-MOTOR NERVES. 3* 



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-dilatators. 



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, katabolic, 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 opportunity 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 raedullated ; 

 the postganglionic 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 method. Nicotine in small doses paralyses 

 nerve-cells,* but not nerve-fibres ; if the drug is injected into an 



* It is still a matter of uncertainty whether this drug acts upon the 

 nerve-cells themselves, or the terminal arborisations (synapses) of the nerve- 

 fibres that surround them. Before the paralytic effect of nicotine comes on, 

 it excites the nerve-cells, and thus causes a general constriction of tin; 

 arterioles and a rise of arterial pressure. 



