216 THE VASCULAR MECHANISM. 



Now when the chorda tympani is simply divided no very remarkable 

 changes take place in the bloodvessels of the gland, but if the peripheral 

 segment of the divided nerve, that still in connection with the gland, be 

 stimulated, very marked results follow. The small arteries of the gland 

 become very much dilated and the whole gland becomes flushed. (As we 

 shall see later on, the gland at the same time secretes saliva copiously, but 

 this does not concern us just now.) Changes in the calibre of the blood- 

 vessels are of course not so readily seen in a compact gland as in a thin, 

 extended ear; but if a fine tube be placed in one of the small veins by 

 which the blood returns from the gland, the effects on the bloodvessels of 

 stimulating the chorda tympani become very obvious. Before stimulation 

 the blood trickles out in a thin slow stream of a dark venous color ; 

 during stimulation the blood rushes out in a rapid full stream, often with a 

 distinct pulsation and frequently of a color which is still scarlet and arte- 

 rial in spite of the blood having traversed the capillaries of the gland ; the 

 blood rushes so rapidly through the widened bloodvessels that it has not 

 time to undergo completely that change from arterial to venous which nor- 

 mally occurs while the blood is traversing the capillaries of the gland. 

 This state of things may continue for some time after the stimulation 

 has ceased, but before long the flow from the veins slackens, the issuing 

 blood becomes darker and venous, and eventually the circulation becomes 

 normal. 



Obviously the chorda tympani contains fibres which we may speak of as 

 " vasomotor," since stimulation of them produces a change in, and brings 

 about a movement in the bloodvessels ; but the change produced is of a 

 character the very opposite to that produced in the bloodvessels of the ear 

 by stimulation of the cervical sympathetic. There stimulation of the nerve 

 caused contraction of the muscular fibres, constriction of the small arteries ; 

 here stimulation of the nerve causes a widening of the arteries, which 

 widening is undoubtedly due to relaxation of the muscular fibres. Hence 

 we must distinguish between two kinds of vasomotor fibres, fibres the stim- 

 ulation of which produces constriction, vaso-constrictor fibres, and fibres 

 the stimulation of which causes the arteries to dilate, vaso-dilator fibres, 

 the one kind being the antagonist of the other. 



The reader can hardly fail to be struck with the analogy between these 

 two kinds of vasomotor fibres on the one hand, and the inhibitory and aug- 

 mentor fibres of the heart on the other hand. The augmentor cardiac fibres 

 increase the rhythm and the force of the heart-beats ; the vaso-constrictor 

 fibres increase the contractions of the muscular fibres of the arteries ; the 

 one works upon a rhythmically active tissue, the other upon a tissue whose 

 work is more or less continuous, but the effect is in each case similar an 

 increase of the work. The inhibitory cardiac fibres slacken or stop the 

 rhythm of the heart and diminish the beats ; the vaso-dilator fibres di- 

 minish the previously existing contraction of the muscular fibres of the 

 arteries so that these expand under the pressure of the blood. 



154. But we must return to the vasomotor nerves. The cervical sympa- 

 thetic contains vaso-constrictor fibres for the ear, and we may now add for 

 other regions, also of the head and face. Thus the branches of the cervical 

 sympathetic, going to the submaxillary gland of which we just spoke (Fig. 

 78, n. sym. sm.\ contain vaso-constrictor fibres for the vessels of the gland ; 

 stimulation of these fibres produces on the vessels of the gland an effect 

 exactly the opposite of that produced by stimulation of the chorda tympani. 

 But to this particular point we shall have to return when we deal with the 

 gland in connection with digestion. A more important fact for our present 

 purpose is that the cervical sympathetic appears to contain only vaso-con- 



