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AN AMERICAN TEXT- BO OK OF PHYSIOLOGY. 



pressor after .section of the splanchnic nerves has little effect, because the 

 blood-pressure is already so low when the stimulation is made that it can sink 

 but little more. When, however, the pressure is restored to its normal level, 

 alter section of the splanchnic nerve- by the stimulation of their peripheral 

 ends, or by the injection of normal saline; solution into the vessels and the 

 depressors then stimulated, the fall in blood-pressure is nearly and some- 

 times quite as great as that obtained by the stimulation of the depressor 

 nerve when the splanchnic nerves are intact. Jt is improbable, therefore, 

 that the depressor acts chiefly through the splanchnic nerves. It probably 

 acts mi all the vasomotor nerves connected with the vasomotor centre. This 

 view is somewhat strengthened by the observations of Bayliss (Fig. 37). 



It has already been said that the depressor fibres pass from the heart to the 

 vaso-motor mechanism in the central nervous system. The cardiac fibres are 

 probably stimulated when the heart is overfilled through lack of expulsive 

 force or through excessive venous inflow, and, by reducing the peripheral resist- 

 ance, assist the engorged organ to empty itself. 



The depressor nerve is not in continual action ; it has no tonus; for the sec- 

 tion of both depressor nerves causes no alteration in the blood-pressure. 



Sewall and Steiner have obtained in some cases a permanent rise in blood- 

 pressure following section of both depressors, yet they hesitate to say that the 

 depressor exercises a tonic action. 



Spallita and Consiglio have stimulated the depressor before and after the 



Fig. 37.— Showing the fall in blood-pressure and the dilatation of peripheral vessels from stimula- 

 tion of the central end of the depressor nerve i Bayliss) : A, curve of blood-pressure in the carotid artery ; 

 B, volume of hind limb, recorded by a plethysmograph ; (7, electro-magnet lino, in which the elevation 

 the time of stimulation of the nerve ; D, atmospheric pressure-line ; E, time in seconds. 



section of the spinal accessory nerve near its junction with the vagus. They 



find that after section of the spinal accessory, the stimulation of the depressor 



does not affect the pulse, whence they conclude that the depressor fibres that 



affect the blood-pressure are separate from those that affect the rate of beat, the 



latter being derived from the spinal accessory nerve. 



A recent study by Bayliss 1 brings out several new facts. If a limb is placed 



1 Bayliss: .J<»inxtl of Physiology, 1893, xiv. p. 303. The relation between the depressor 

 nerve and the thyroid i- pointed nut by v. ( 'von : ( ' ,.- • .' \ttfur Physiologic, 1897, ii. pp. 279, 357. 



