328 



VASO-MOTOR FIBRES OF THE EAR. [BOOK i. 



G.C.S 



ments of division and stimulation in a series of animals, we may 

 trace the path of these impulses from the lower cervical ganglion, 

 Fig. 73, through the annulus of Vieussens to 

 i - \r/ the ganglion stellatum and upper part of the 

 thoracic sympathetic chain, and thence along 

 the rami communicantes of some or other 

 of the upper thoracic spinal nerves to the 

 anterior roots of those nerves and so to the 

 spinal cord. In the cat and the dog, and 

 probably in other higher mammals, the chief 

 path of the impulses lies in the second and 

 third thoracic nerves, though some pass by 

 the fourth and a variable small number by 

 the fifth and the first ; in the rabbit the 

 path is more widespread and reaches lower 

 down, for while the impulses pass chiefly by 

 the fourth and fifth thoracic nerves, some 

 pass by the second and third, and others by 

 the sixth, seventh and even eighth nerves. 

 The exact path also differs in different indi- 

 viduals of the same species. It will be 

 observed that from the spinal cord up to the 

 annulus of Vieussens, and the lower cervical 

 ganglion, these vaso-motor impulses for 

 the ear, and the augmentor impulses for the 

 heart, (cf. Fig. 72) follow much the same 

 path ; but there they part company. We 



FIG. 73. DIAGRAM ILLUSTRATING THE PATHS OF VASOCONSTRICTOR FIBRES ALONG 

 THE CERVICAL SYMPATHETIC AND (PART OF) THE ABDOMINAL SPLANCHNIC. 



Aur. artery of ear. G.C.S. superior cervical ganglion. Abd. Spl. upper roots 

 of and part of abdominal splanchnic nerve. V. M. C. vaso-motor centre in spinal 

 bulb. The other references are the same as in Fig. 72, 160. The paths of the 

 constrictor fibres are shewn by the arrows. The dotted line along the middle of 

 the spinal cord, Sp.C., is to indicate the passage of constrictor impulses down 

 the cord from the vaso-motor centre in the spinal bulb. 



can thus trace these vaso-motor impulses backwards along the cer- 

 vical sympathetic to the anterior roots of certain thoracic nerves, and 

 through these to the thoracic region of the spinal cord, where we 

 will for the present leave them. We may accordingly speak of 

 vaso-motor fibres for the ear as passing from the thoracic spinal 

 cord to the ear along the track just marked out ; stimulation of these 

 fibres at their origin from the spinal cord or at any part of their 

 course (along the anterior roots of the second, third or other upper 

 thoracic nerves, visceral branches (rami communicantes) of those 

 nerves, ganglion stellatum or upper part of thoracic sympathetic 

 chain, annulus of Vieussens, &c. &c.) leads to constriction in the 

 blood vessels of the ear of that side ; and section of these fibres 

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



