466 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 l brings out several new facts. If a limb is placed 

 in Mosso's plethysmograph and the central end of the depressor stimulated, 

 the volume of the limb increases, showing an active dilatation of the vessels 

 that supply it. The latent period of this dilatation varies greatly. The vessels 

 of the skin play a large part in its production. A similar local action is seen 

 on the vessels of the head and neck (see Fig. 122). 



The depressor fibres vary much in size in different animals. When the 

 nerve is small, a greater depressor effect can be obtained by stimulating the 

 central end of the vagus than from the depressor itself. But the course of the 

 fall is different in the two cases. With the depressor, the fall is maintained at 

 a constant level during the whole excitation, however long it lasts, whereas 

 in the case of the vagus the pressure very soon returns to its original 

 height although the excitation still continues. Bayliss believes, therefore, 

 that there is a considerable difference between the central connections of 

 the depressor nerve itself and the depressor fibres sometimes found in other 

 nerves. 



The left depressor nerve usually produces a greater fall of pressure than the 

 right. The excitation of the second nerve during the excitation of the first 

 produces a greater fall than the excitation of one alone. 



The fibres of the depressor, in part at least, end in the wall of the ventricle. 2 

 A similar nerve has been demonstrated in the cat, 3 horse, 4 dog, 5 sheep, 6 swine, 7 

 and in man. 8 



Sensory Nerves. The first and usually the only effect of the stimulation 

 of the central end of a mixed nerve like the sciatic, according to Roy and 

 Adami, 9 is an increase in the force and the frequency of the heart-beat. Other 

 observers 10 have sometimes found quickening and sometimes slowing of the pulse- 

 rate, so that sensory nerves, as Tigerstedt n suggests, appear to affect both the 

 inhibitory and the augmenting heart-nerves. When a sensory nerve is weakly 

 excited the augmentor effect predominates, when strongly excited the inhibi- 

 tory. A well-known demonstration of the reflex action of the sensory nerves 

 on the heart is seen in the slowing of the rabbit's heart when the animal 



1 Bayliss, 1893, p. 304. 2 Kazem-Beck, 1888, p. 329. 



3 Bernhardt, 1868, p. 5 ; Aubert and Koever, 1868, p. 214 ; Kowalewsky and Adamiik, 1868, 

 p. 545 ; Koever, 1869, p. 68 ; Kazem-Beck, 1888, p. 331. 



* Bernhardt, 1868, p. 5 ; Cyon, 1870, p. 262 ; Finkelstein, 1880, p. 350. 



6 Koever, 1869, p. 71 ; Langenbacher, 1877 ; Kreidmann, 1878, p. 411 ; Finkelstein, 1880, p. 

 248 ; Kazem-Beck, 1888, p. 332. 



6 Kriedmann, 1878, p. 407. 



7 Langenbacher, 1877 ; Kazem-Beck, 1888, p. 335 ; the latter describes also (p. 338) a de- 

 pressor nerve in cold-blooded animals ; compare Gaskell and Gadow, 1885, p. 362. 



8 Bernhardt, 1868, p. 5; Kreidmann, 1878, p. 408; Finkelstein, 1880, p. 249; Be*ke"sy, 1888. 



9 Koy and Adami, 1892, p. 254. 



10 LoveX 1866, p. 5 ; Bernard, 1858, p. 291 ; Asp, 1867, p. 173 ; Tranck, 1876, p. 246 ; Siman- 

 owsky, 1881. "-Tigerstedt, 1893, p. 287. 



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