214 



THE VASCULAR MECHANISM. 



FIG. 77. 



V.M.C.. 



Sp.C. 



upward to the head, become connected with the arteries of the ear, and that 

 these fibres are of such a kind that impulses generated in them and passing 

 upward to the ear lead to marked contraction of the muscular fibres of the 

 arteries, and thus produce constriction. These fibres are vasomotor fibres 

 for the bloodvessels of the ear. From the loss of tone, so frequently follow- 

 ing section of the cervical sympathetic, we may further infer that, normally 

 during life, impulses of a gentle kind are continually passing along these 

 fibres upward through the cervical sympathetic, which impulses, reaching 

 the arteries of the ear, maintain the normal tone of those arteries. But, as 

 we said, the existence of this tone is not so constant, and these tonic impulses 

 are not so conspicuous as the artificial constrictor 

 impulses generated by stimulation of the nerve. 



151. The above results are obtained what- 

 ever be the region of the cervical sympathetic 

 which we divide or stimulate from the upper cervi- 

 cal ganglion to the lower. We may, therefore, de- 

 scribe these vasomotor impulses as passing upward 

 from the lower cervical ganglion along the cervical 

 sympathetic to the upper cervical ganglion, from 

 which they issue by branches which ultimately 

 find their way to the ear. But these impulses do 

 not start from the lower cervical ganglion ; on 

 the contrary, by repeating the experiments of di- 

 vision and stimulation in a series of animals, we 

 may trace the path of these impulses from the 

 lower cervical ganglion (Fig. 77) through the 

 annulus of Vieussens to the ganglion stellatum or 

 first thoracic ganglion, and thence either along the 

 ramus communicans (visceral branch) to the an- 

 terior root of the second dorsal nerve, and thus to 

 the spinal cord, or lower down along the thoracic 

 sympathetic chain, and thence by other rami com- 

 municantes to some other of the upper dorsal 

 nerves, and thus to the spinal cord. The path 

 taken by these vasomotor impulses for the ear is 

 in fact very similar to that of the augmentor fibres 

 for the heart (cf. Fig. 76), from the spinal cord 

 up to the annulus of Vieussens and to the lower 

 cervical ganglion ; but there they part company. 

 We can thus trace these impulses along the cer- 

 vical sympathetic to the anterior roots of certain 

 dorsal nerves, and through these to a particular 

 part of the spinal cord, where we will for the pres- 

 ent leave them. We may accordingly speak of 

 vasomotor fibres for the ear as passing from the 

 dorsal spinal cord to the ear along the track just 

 marked out ; stimulation of these fibres at their 

 origin in the spinal cord or at any part of their 

 course (along the anterior roots of the second, 

 third, or other upper dorsal nerves, visceral 

 branches of those nerves, ganglion stellatum or 

 upper part of thoracic sympathetic chain, annulus 

 of Vieussens, etc.) leads to constriction in the 

 bloodvessels of the ear of that side ; and section 

 of these fibres at any part of the same course tends to abolish any previously 



Diagram illustrating the 

 Paths of Vasoconstrictor Fi- 

 bres along the Cervical Sym- 

 pathetic and (part of) the Ab- 

 dominal Splanchnic : Aur., 

 artery of ear ; G. C.s., superior 

 cervical ganglion ; Abd. Spl., 

 upper roots of and part of ab- 

 dominal splanchnic nerve ; V. 

 M. 0., vasomotor centre in me- 

 dulla. The other references 

 are the same as in Fig. 76, 1 

 146. The paths of the constric- 

 tor fibres are shown by the 

 arrows. The dotted line in 

 the spinal cord, Sp. C., is to in- 

 dicate the passage of constric- 

 tor impulses down the cord 

 from the vasomotor centre in 

 the medulla. 



