THE DEPRESSOR NERVE. 61 



An alternative view is that the depressor excites a hypothetical 

 vaso-dilator centre. There are, however, certain differences between the 

 depressor and other vaso-dilator nerves. The latter are easily exhausted 

 by artificial stimulation, while the former is inexhaustible. On the other 

 hand, according to Bayliss, section of the spinal cord at the level of 

 the second lumbar nerve-root 1 prevents, when the depressor is excited, 

 any manifestation of dilatation in the lower limbs ; and fibres pro- 

 ducing vasodilatation of the legs have been found, by Bradford and 

 Bayliss, by direct excitation of the third, fourth, fifth, sixth, and 

 seventh lumbar roots. The question is one not yet definitely settled, 

 and no conclusion as to the exact method of action of this nerve can 

 be drawn. Light should be thrown on the subject by experiments on 

 such a gland as the salivary, which possesses a distinct and powerful 

 supply of both vaso-dilators and vaso-constrictor nerves. It should be 

 possible to determine by section of one or other of these nerves the 

 channel by which the depressor acts. The depressor stands alone as a 

 distinct type of nerve. No other nerve in the body can, on excitation 



Fig. 41. — Aortic pressure and volume curve of limb. A, Excitation 

 of depressor. — Bayliss. 



of the central end, uniformly produce a reflex fall of blood pressure. 

 By all other nerves, either pressor or depressor effects are provoked, 

 and, generally, on prolonging the excitation, the first effect is followed 

 by the second. The depressor is purely an afferent dilator nerve. 



Almost all parts of the vascular system can be thrown into dilata- 

 tion by the depressor nerve. While compression of the thoracic aorta 

 does not entirely abolish the fall of arterial tension produced by excita- 

 tion of this nerve, section of the spinal cord in the cervical region does 

 do so. It follows that parts of the body above the abdomen must share 

 in the reflex dilatation. 



By Bayliss the question has been minutely studied. He employed 

 the plethysmography method. The limbs were enclosed in a glass 

 cylinder, the connection being made air-tight by means of an india- 

 rubber collar greased with vaselin ; the variations of volume were 

 recorded by means of a tube passing from the cylinder to a delicate 

 tambour. In such experiments, in order to avoid disturbing elements, 

 due to movements or cardiac inhibition, it is necessary to place the 

 animal under curari, and to divide both vagi. The limbs dilate on 

 exciting the depressor, but the latent period is long. 2 This is in con- 

 sequence of the primary passive constricting effect produced in the 

 limb by the fall of arterial tension. 



1 The vaso-constrictor nerves for the lower limb come off above this level. 



2 Bayliss, Journ. Physiol., Cambridge and London, 1S93, vol. xiv. p. i!04. 



