494 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



motor centres exist in the brain aside from the bulbar centres already described. 

 At present the safer view is that the changes in blood-pressure called forth by 

 the stimulation of various parts of the brain are reflex actions, the afferent im- 

 pulse starting in the brain as it might in any other tissue peripheral to the 

 vaso-motor centres. 1 



Pressor and Depressor Fibres. The stimulation of the same afferent 

 nerve sometimes causes reflex dilation of the vessels of a part, instead of the 

 more usual reflex constriction. Two explanations of this fact have been sug- 

 gested. The first assumes that the condition of the vaso-motor centre varies in 

 such a way that the same stimuli might produce contrary effects, depending 

 on the relation between the time of stimulation and the condition of the centre. 

 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, 2 Ho well, 3 and others. Howell, for example, has found that if a part of 

 the sciatic nerve is cooled to near 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, 4 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 5 and 

 by 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. 6 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 



1 Literature: Dogiel, 1880, p. 420; Strieker, 1886, p. 9; Bechterew and Mislawsky, 1886, 

 p. 193; Franck, 1887, p. 162. 



2 Latschenberger and Deahna, 1876, p. 165. 



3 Howell, Budgett, and Leonard, 1894, p. 310. Other literature : Belneld, 1882, p. 298 ; Knoll, 

 1885, p. 447, 1889, p. 249 ; Kleen, 1887, p. 247 ; Bayliss, 1893, p. 317 ; Bradford and Dean, 

 1894, p. 67 ; Hunt, 1895, p. 381. 



* Hunt, 1895, p. 381. 



6 See also Knoll, 1885, p. 451. 



6 Literature : Mosso, 1880, p. 1-127 ; Franck, 1887, p. 199 ; Gaertner and Wagner, 1887, p. 602 ; 

 Corin, 1888, p. 185 ; Hiirthle, 1889, p. 561 ; Koy and Sherrington, 1890, p. 85 ; Cavazzani, 1891, 

 p. 23 ; 1893, pp. 54, 214 ; Bayliss and Hill, 1895, p. 334 ; Gulland, 1895, p. 361. 



