FUNCTIONS OF THE SYMPATHETIC. 109 



carotids, which anastomose very freely with each other, to obviate 

 the effects of pressure upon either of them. Their course is also a 

 tortuous one, that the impulse of the blood upon so delicate an organ 

 may be avoided. 



A certain amount of regulated pressure upon the walls of the 

 vessels of the brain is essential to the proper exercise of its func- 

 tions. It is known that the amount of blood circulating through 

 the brain varies at different times in accordance with its in- 

 creased or diminished functional activity, and that the cranial case 

 is not an unyielding one, as its contents would then be invariable. 

 A special provision is made to meet this varying amount of fluid, 

 and to keep up the same degree of pressure upon the organ, in the 

 existence of a fluid beneath the arachnoid wherever the pia mater 

 exists. The amount of this fluid averages about 2 ounces, but in 

 cases of atrophy of the brain, as much as 12 ounces may be ob- 

 tained from the cranio-spinal cavity ; whilst in all instances in 

 which the bulk of the brain has undergone an increase, either from 

 tlie production of additional nervous tissue, or from undue turgescence 

 of the vessels, there is either a diminution, or a total absence of this 

 fluid. It has been shown by Magendie that the withdrawal of this 

 fluid in living animals always causes great disturbance of the 

 cerebral functions, probably by allowing undue distension of the 

 blood-vessels. It is, however, speedily renewed, and its reproduction 

 restores the nervous centres to their normal condition. 



FUNCTIONS OF THE SYMPATHETIC. 



This nerve has been variously called the splancJmic^ ganglionic, 

 vegetative^ and organic. It consists of an uninterrupted chain of 

 ganglia, extending on each side of the vertebral column, from the 

 first cervical vertebra down to the coccyx, and moreover extending 

 upwards beside the cranial vertebrse, and occupying spaces between 

 the bones of the cranium and those of the face. The chains of the 

 opposite sides communicate with each other at various points in the 

 plexuses of nerves that originate from them, and through the gan- 

 glion impar^ a single ganglion in front of the coccyx. A cephalic 

 communication is as yet uncertain. Ribes described a ganglion 

 impar situated upon the anterior commrinicating artery of the 

 circle of Willis, and other anatomists regard the pituitary body in 

 the sella turcica as a ganglion of like description, a common point 

 of union for the right and left chains at their cranial extremities. 



The peculiarities of the sympathetic fibres were spoken of when 

 describing the development of the nervous system, (page 89.) 



The sympathetic system contains two sets of nervous fibres, — 

 the ordinary white tubular fibres, which it derives from the cerebro- 

 spinal system ; and the gray or gelatinous fibres, which belong ex- 

 clusively to itself. 



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