242 A TEXTBOOK OF PHYSIOLOGY 



outside world demands cerebral response, the splanchnic area constr.'cts r 

 and more blood is driven through the brain. 



The arterial supply to the brain is in the lower animals so super- 

 abundant that in the dog four of the arteries two common carotids 

 and two vertebrals which supply the brain can be tied in the course 

 of ten minutes, and yet the animal either at once, or after a temporary 

 period of idiocy and paralysis, completely recovers. In the monkey, 

 on the other hand, ligation in one operation of the two carotids and 

 one vertebral artery produces either death in twenty-four hours or 

 softening of the great brain, accompanied by idiocy and paralysis 

 of movement and sensation. The efficiency of anastomosis through 

 the circle of Willis varies in different individuals. Sudden com- 

 pression of one common carotid artery in some men produces epileptic 

 spasm, and ligation of this artery has been followed in some by more 

 or less temporary paralysis on the opposite side of the body ; in others 

 the effect is nil. 



Whether the cerebral arteries are supplied with vaso-motor nerves 

 is very doubtful. The usual methods of investigation give no 

 evidence of these in the brain of the intact animal. After 

 establishing an artificial circulation of the brain, the addition of 

 adrenalin to the nutritive fluid is stated to reduce the outflow; and 

 it is supposed that adrenalin acts by stimulating the ends of the vaso- 

 motor nerves, rather than by stimulating the muscular coats of the 

 arteries. The veins of the pia and dura mater have no middle muscular 

 coats and no valves. The venous blood emerges from the skull in 

 man mainly through the opening of the lateral sinuses into the internal 

 jugular vein; there are communications betweeii the cavernous sinuses 

 and the ophthalmic veins of the facial system, and with the venous 

 plexuses of the spinal cord. The points of emergence of the veins 

 are well protected from closure by compression. The brain can regulate 

 its own blood-supply by means of the cardiac and vaso-motor centres. 

 Deficient supply to these centres excites increased frequency of the 

 heart and constriction of the arteries, especially those of the great 

 splanchnic area. Cerebral excitement has the same effect, so that 

 the active brain is assured of a greater blood-supply. As the brain 

 in the processes of ideation, etc., acts as a whole, vaso-motor nerves- 

 for locally controlling the cerebral circulation seem unnecessary. 



The Circulation in the Head. The vaso-constrictor fibres for the 

 head issue from the spinal cord by the upper thoracic anterior roota 

 (1 to 5 in the dog), and pass into the cervical sympathetic nerve. 



The vaso-dilator supply to the face and mouth also issues from 

 the upper thoracic roots, and passes up the cervical sympathetic nerve 

 to the Gasserian ganglion, and thence to the fifth nerve. Some of 

 the dilator fibres issue directly from the cranial origin of the fifth 

 nerve, for stimulation of this nerve between the pons and the Gasserian 

 ganglion flushes the face. Excitation of the lingual and glosso- 

 pharyngeal nerves dilates the vessels of the tongue, while stimulation 

 of the hypoglossal nerve constricts that organ. These constrictor 

 fibres come from the superior cervical sj^mpathetic ganglion. 



