146 INTRODUCTION TO GENERAL PHYSIOLOGY 



driving pressure will be greater. This is one way in which the 

 supply to any particular organ may be increased, and so far as 

 investigation has hitherto been able to make out, it is the only way 

 in which that to the brain is regulated, so far as other than chemical 

 factors acting, directly on the blood vessels are concerned. If the 

 brain requires more blood, the whole of the rest of the body has to 

 put up with less. In most organs, however, the arterioles have the 

 power of widening in response to messages from the central nervous 

 system, thus ensuring for each particular organ the more copious 

 supply that is wanted when it enters into activity. There is thus 

 a double supply of nerves to the muscle of the arterioles as to 

 smooth muscle in general, one set of nerves exciting to increased 

 contraction, the other inhibiting the natural tone. The former are 

 called " vaso-constrictor " nerves, and all leave the central nervous 

 system in the sympathetic outflow (E., p. 224). The latter are 

 " vaso-dilator " nerves, and have a more various origin. A familiar 

 case of reflex vaso-dilatation is the reddening of the skin known 

 as "blushing." Each set has a governing centre, the source of 

 reflexes to blood vessels, in that part of the brain immediately at 

 the upper end of the spinal cord, called the "bulb" or "medulla 

 oblongata." Certain sensory nerves produce, on stimulation, a 

 reflex fall of blood pressure by general dilatation of the arterioles ; 

 others a rise by general vaso-constriction. The former are some- 

 times called " depressor " and the latter " pressor" reflexes. 



The vaso-constrictor centre is normally sending out impulses in 

 a continuous stream down the spinal cord and through the 

 sympathetic to the blood vessels, so that these are kept in a state 

 of partial contraction. When a depressor reflex is produced, the 

 vaso-constrictor centre is inhibited, while the vaso-dilator centre is 

 excited. In fact, we have "reciprocal innervation " of a rather 

 more complex kind than in that of reflexes to voluntary muscles. 

 The converse effect in a pressor reflex is not so easy to show, since 

 the vaso-dilator centre does not send out a steady discharge except 

 under special circumstances. The excitation of the constrictor 

 centre is easily to be made out. 



One particular reflex requires mention, that from the nerve 

 which has received the special name of " depressor? The receptor 

 endings are situated chiefly in the beginning of the aorta, so that 

 when the blood pressure rises too high a dilator reflex is sent to 

 the arterioles in general, and the pressure lowered. 



Another kind of vascular reflex is met with in several organs, 

 and is probably of wide occurrence. It is known as the " Loven 

 reflex" from the Swedish physiologist who first described it. When 

 a sensory nerve passing from the rabbit's ear, for example, is 

 stimulated so as to evoke a reflex to the blood vessels, it is found 



