CIRCULA TION. 1 7 5 



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. 37). 



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 dui'ing 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. 

 A similar nerve has been demonstrated in the cat, horse, dog, sheep, swine, 

 and in man. 



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

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

 rate, so that sensory nerves, as Tigerstedt 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 

 is made to inhale chloroform. The superior laryngeal and the trigeminus 

 nerves, especially the latter, convey the stimulus to the nerve-centres. 



The stimulation of t he nerves of special sense, optic, auditory, olfactory and 

 glosso-pharyngeal nerves, also sometimes slows and sometimes quickens the 

 heart. 



Sympathetic. — The reflex action of the sympathetic nerve upon the heart 

 is well shown by the celebrated experiment of F. Goltz. In a medium-sized 

 frog, the pericardium was exposed by carefully cutting a small window in the 

 chest-wall. The pulsations of the heart could be seen through the thin peri- 

 cardial membrane. Goltz now began to heat upon the abdomen aboul 1 10 

 times a minute witli the handle of a scalpel. The heart gradually slowed, and 

 at length stood still in diastole. Goltz now ceased the rain of little blows. 

 The heart remained quiet for a time and then began to beat again. ;it firsl slowly 

 and then more rapidly. Some time after the experiment, the heart beal about 

 five strokes in the minute faster than before the experiment was begun. The 

 effect cannot be obtained after section of the vaffi. 



