THE SYMPATHETIC SYSTEM. 505 



with the pupil and eyeball follow in the same way as if the sympathetic 

 had been cut in the neck, but there is no vascular congestion or in- 

 creased temperature of the parts ; and galvanization of the peripheral 

 extremities of the divided nerve roots, causes dilatation of the pupil, 

 like galvanization of the sympathetic in the neck. On the other 

 hand, if the trunk of the sympathetic be divided in the upper part of 

 the chest, between the heads of the second and third ribs, there is no 

 contraction of the pupil, but the temperature of the ear is increased 

 from 4 to 6 C. above that of the opposite side. 



There is accordingly a remarkable difference between the nerve fibres 

 for sensation and voluntary motion and those for the blood-vessels in 

 the route which they follow to their distribution in the head. The 

 sensitive and motor nerves of the head and face emerge from the base 

 of the brain and pass, through the cranial foramina, to the integument 

 and muscles. Those destined for the blood-vessels are given off from 

 the spinal cord, mainly with the roots of the third pair of dorsal nerves, 

 whence they join the sympathetic, passing upward through its cervical 

 portion to the head and face. 



There is also a difference of origin, though less marked, between the 

 fibres for sensation and volition and those for vasomotor action in the 

 limbs. The vasomotor fibres for the upper limb do not originate 

 with the nerve roots going to form the brachial plexus, but farther 

 down, in the dorsal portion of the cord. Bernard found that, in the 

 dog, division of the last three cervical and first two dorsal nerves within 

 the spinal canal causes paralysis of motion, and sensation in the cor- 

 responding foreleg, but no vascular congestion or calorification. On 

 the other hand, extirpation of the first thoracic ganglion of the sym- 

 pathetic, or section of the nerves of the brachial plexus after they have 

 been joined by filaments from this ganglion, causes an elevation of tem- 

 perature in the corresponding limb ; and the samo result follows divi- 

 sion of the thoracic portion of the sympathetic between the third and 

 fourth dorsal vertebrae. The vasomotor fibres paralyzed by this section 

 come therefore from below the first thoracic ganglion ; and, according 

 to Cyon, they emanate from the spinal cord with the roots of the dorsal 

 nerves, from the third to the seventh pairs inclusive. 



The vasomotor fibres for the lower limb have a similar origin. Ac- 

 cording to Bernard, section of the spinal nerve roots destined for the 

 lumbo-sacral plexus, in the dog, paralyze the corresponding hind leg 

 without causing increase of temperature ; but the latter effect is pro- 

 duced in addition by dividing the sympathetic at the level of the fifth 

 and sixth lumbar vertebrae, or by section of the sciatic nerve. The 

 vasomotor fibres of the limbs are therefore distinct from those which 

 supply them with ordinary motion and sensibility. 



Tonic Contraction of Blood-vessels and its Influence on the Circu- 

 lation. Under the stimulus of the sympathetic fibres distributed to 

 the arterial walls, the vessels are normally maintained in a moderate 

 state of contraction. This continuous muscular activity is the " tone " 

 or tonic contraction of the arteries, by which they offer a certain resist- 



