CIRCULATION. 203 



The second assumes the existence of special reflex constrictor or "pressor" 

 fibres, and reflex dilator or "depressor" fibres. The existence of at least one 

 depressor nerve is beyond question, namely the cardiac depressor nerve, which 

 it will be remembered runs from the heart to the bulb and when stimulated 

 causes a dilatation of the splanchnic and other vessels reflexly through the 

 bulbar vaso-motor centre. Evidence of other reflex vaso-dilator nerves and of 

 reflex vaso-constrictor fibres as well has been offered by Latschenberger and 

 Deahna, Howell, 1 and others. Howell, for example, has found that if a part of 

 the sciatic nerve is cooled to near 0° C. and the central end stimulated periph- 

 erally to this part, the blood-pressure falls, instead of rising, as it does when 

 the nerve is stimulated without previous cooling. Howell's experiments have 

 been recently extended by Hunt, who finds that the stimulation of the sciatic 

 during its regeneration after section gives at first vaso-dilatation only, but when 

 regeneration has progressed still further, vaso-constriction is secured. These 

 results point to the existence of both pressor and depressor fibres, the latter 

 being the first to regenerate after section. A reflex fall in blood-pressure is 

 also produced by stimulating various mixed nerves with weak currents and 

 bv the mechanical stimulation of the nerve-endings in muscle. The fall is 

 more readily obtained when the animal is under ether, chloroform, or chloral, 

 less readily under curare. 



Topography. — We pass now to the vaso-motor nerves of various regions. 



Brain. 2 — The study of the innervation of the intracranial vessels is ren- 

 dered exceptionally difficult by the fact that the brain and its blood-vessels are 

 placed in a closed cavity surrounded by walls of unyielding bone. The funda- 

 mental difference created by this arrangement between the vascular phenomena 

 of the brain and those of other organs was recognized in part at least by 

 the younger Monro as long ago as 1783. Monro declared that the quantity 

 of blood within the cranium is almost invariable, " for, being enclosed in a 

 case of bone, the blood must be continually flowing out of the veins that room 

 may be given to the blood which is entering by the arteries, — as the substance 

 of the brain, like that of the other solids of our body, is nearly incompress- 

 ible." Further differences between the circulation in the brain and in other 

 organs are introdueed by the presence of the cerebrospinal fluid in the ventri- 

 cles and in the arachnoidal spaces at the base of the brain. This fluid may pass 

 out into the spinal canal and thus leave room for an increase in the amount 

 of blood in the cranium. Finally, a rise of pressure in the arteries too great 

 to be compensated by the outflow of cerebro-spinal fluid may lead to com- 

 pression of the venous sinuses and a decided change in the relative distri- 

 bution of the blood in the arteries, capillaries and vein — conditions which are 

 not present in extracranial tissues. It is evident, therefore, thai the methods 

 employed in the search for vaso-motor nerves within the cranium must take 



'Howell, Budgett, and Leonard Journal of Physiology, 1894, xvi. p. .'<U> : Bayliss: Ibid., 

 1893, xiv. p. 317 ; Bradford and Dean: Ibid,, 1894, xvi. p. 67 ; Hunt: Ibid., 1895, xviii. p. 381. 



1 Cavazzani: Archives italiennes de Biologie, 1893, xviii. p. 54, lix. p. 214; Bayliss and Hill: 

 Journal of Physiology, 1895, xviii. p. 334; Gulland: Ibid., p. 361. 



