230 ESSENTIALS OF PHYSIOLOGY. 



sensory nerve B into the spinal cord; in its course each nerve fibre 

 gives off a branch C, which ends in the walls of the arterioles (D) of 

 the limb. The impulse passing along the fibre B also passes along C 

 and relaxes the muscle of the arterioles. Since nerve fibres can con- 

 duct impulses in both directions, stimulation of the posterior root 

 fibres at E gives rise to an impulse which, travelling down the nerve, 

 passes by the branch C to the arterioles at D, and causes them to relax. 

 We see, therefore, that whether the stimulus is applied at the periphery 

 A or at E, the impulse reaches the arterioles along the branch C. The 

 effect of stimulation at A, which is not a true reflex, is called an axon 

 reflex. If the posterior root fibres become degenerated peripherally to 

 the ganglion, the axon reflex disappears, and a stimulus applied to the 

 skin at A causes no dilatation of the subcutaneous vessels. 



This reflex is of great importance to the body. As is well known, 

 an irritant (e.g. a mustard blister) applied to the skin causes dilatation 

 of the cutaneous vessels and reddening of the skin. The dilatation of 

 the vessels is part of the means by which the tissues protect themselves 

 against injuries or irritants, and if the vascular changes do not occur, 

 owing to degeneration of the peripheral sensory fibres, the damage done 

 by the irritant to the tissues may be much more severe. 



Vaso-dilator fibres are also present in the sympathetic system itself, 

 although their presence is not readily demonstrated owing to the greater 

 abundance of vaso-constrictor fibres ; but when the endings of the latter 

 are paralysed by the drug ergotoxin, stimulation of the splanchnic 

 nerves causes vaso-dilatation and a fall of blood pressure. 



The vaso-dilator fibres seem to be concerned mainly, though not 

 entirely, with bringing about an increased flow of blood in individual 

 organs, whereas the vaso-constrictor fibres, controlled by the vaso-motor 

 centre, regulate the tone of the arterioles of the body as a whole. 



Influences Affetting the Vaso-motor Centre. The vaso-motor centre 

 is extremely susceptible both to impulses reaching it from other parts 

 of the nervous system, whether these reach it from the higher parts of 

 the brain or from the peripheral nerves, and to changes in the character 

 and amount of the blood passing to the brain. Its activities are con- 

 stantly varying in response to these stimuli, in such a way that .the 

 mean arterial pressure is raised to meet special needs of the body, and 

 is prevented from falling below the level necessary for the adequate 

 supply of blood to the tissues, and more especially to the brain. 



(1) Nervous Stimuli. The depressor nerve is a purely afferent 

 nerve, originating in the root of the aorta, and passing to the brain. 

 Electrical stimulation of its central end causes a lessening of the activity 

 of the vaso-motor centre, decrease of the tone of the arterioles, and a 



