CEPHALO-SPINAL FLUID. 107 



inosculate with corresponding branches of the carotids, and form a kind 

 of circle at the base of the brain, which has been called circulus arte- 

 riosus of Willis. The passage of the blood-vessels is extremely tortu- 

 ous, so that the blood does not enter the brain with great impetus ; and 

 they become capillary before they penetrate the organ, an arrange- 

 ment of importance, when we regard the large amount of blood sent 

 to it. This has been estimated as high as one-eighth of the whole fluid 

 transmitted from the heart. The amount does not admit of accurate 

 appreciation, but it is considerable. It of course varies according to 

 circumstances. In hypertrophy of the heart, the quantity is sometimes 

 increased ; as well as in ordinary cases of what are called determinations 

 of blood to the head. Here, too large an amount is sent by the arterial 

 vessels ; but an equal accumulation may occur, if the return of the 

 blood from the head by the veins be in any manner impeded, as when 

 we stoop, or compress the veins of the neck by a tight cravat, or by 

 keeping the head turned for a length of time. Congestion or accumu- 

 lation of blood may therefore arise from very different causes. 



Sir Astley Cooper 1 found by experiment, that the vertebral arteries 

 are more important vessels as regards the encephalon and its functions 

 in certain animals, as the rabbit, than the carotids. The nervous power 

 is lessened by tying them ; and, in his experiments, the animals did 

 not, in any case, survive the operation more than a fortnight. In the 

 dog, he tied the carotids with little effect, but the ligature of the verte- 

 brals had a great influence. The effect of the operation was to render 

 the breathing immediately difficult and laborious ; owing, in Sir Astley's 

 opinion, to the supply of blood to the phrenic nerves, and the whole 

 tractus respiratorius of Sir Charles Bell, being cut off. The animal v 

 became dull, and indisposed to make use of exertion ; or to take food. 

 Compression of the carotids and the vertebrals at the same moment, 

 in the rabbit, destroyed the nervous functions immediately. This was 

 effected by the application of the thumbs to both sides of the neck, the 

 trachea remaining free from pressure. Respiration ceased entirely, 

 with the exception of a few convulsive gasps. The same fact was 

 evinced in a clearer and more satisfactory manner by the application 

 of ligatures to the four vessels, all of which were tightened at the same 

 instant. Stoppage of respiration and death immediately ensued. 



The cerebral, like other arteries, are accompanied by branches of the 

 great sympathetic. The researches of Purkinje, 2 Volkmann, 3 and 

 Rainey, 4 have shown the existence of a large number of nerves in con- 

 nection with the encephalic and spinal arachnoid. They do not seem to 

 communicate with the roots of the spinal nerves, but belong exclusively 

 to the sympathetic. 5 The encephalic veins are disposed as already de- 

 scribed, terminating in sinuses formed by the dura mater, and conveying 



1 Guy's Hospital Reports, i. 472, London, 1836. 



2 JMulIer's Archiv. fur Anatomie, p. 281, Berlin, 1845. 



3 Art. Nervenphysiologie, Wagner's Handworterbuch der Physiologie, lOte Lieferung, 

 s. 494, Braunschweig, 1845. 



< Medico-Chirurgical Transactions for the year 1845. 



6 D. Brinton, Art. Serous and Synovial Membranes, in Cyclop, of Anat. and Physiol., Pt. 

 xxxiv. p. 525, Lond., Jan. 1849. 



