620 SYMPATHETIC AND OTHER SYSTEMS OF NERVES. 



cervical sympathetic is cut, and the stimulation repeated, in order to 

 make certain that the effects were due to impulses passing by this nerve. 

 It is thus found that the different classes of fibres have different origins ; 

 they differ also, though not very greatly, in different animals. 



The pupillo-dilator fibres arise from the first three thoracic nerves. 

 In the cat and dog the third thoracic has much less effect than the other 

 two. On the other hand, in the rabbit, and apparently also in the ape, 

 the third thoracic may have a greater effect than the first, though never 

 so great as the second thoracic nerve. 



The nerve fibres for the nictitating membrane, the eyelids, and the 

 muscles of the orbit, arise from a greater number of nerves than those 

 for the pupil, and their origin is on the whole more posterior. They 

 arise in the cat and dog from the first five thoracic nerves ; but the fifth 

 thoracic has a very slight effect, and the fourth less than the first three. 

 Of the first three, the second and third are more effective than the first. 



The vaso-constrictor and vaso-dilator fibres are again, as a whole, a 

 little more posterior in origin than the fibres supplying the nictitating 

 membrane. They arise from the second to the fifth thoracic nerves 

 inclusive, and occasionally a few from the first ; the maximum effect is 

 obtained from the third, or from the third and fourth. In the rabbit 

 the origin of the vasomotor fibres to the ear is more extensive; some 

 pass by the sixth and seventh, and occasionally a few by the eighth 

 thoracic. It is not, however, certain whether these fibres of lower 

 origin run to the cervical sympathetic, or make their way by the ramus 

 vertebralis of the ganglion stellatum (cf. p. 624). 



The submaxillary gland of the dog receives fibres also from the 

 second to the fifth nerves inclusive, but the maximum effect is obtained 

 from the second thoracic. 



Lastly, we have the pilo-motor nerves. Those in the cat are of 

 much lower origin than the others. They arise from the fourth to the 

 seventh thoracic nerves inclusive, and the maximum effect is obtained 

 from the fifth and sixth. 1 



It will facilitate a general view of these facts to put those which 

 are best established in a tabular form : — 



Effects of Stimulation of Nerve Roots, indicating Origin of Fibres of the 

 Cervical Sympathetic in the Cat. 



1 In the ape, Sherrington found the pilo-motor fibres for the head to issue by the second 

 to the fifth thoracic nerves. 



