VASOMOTOR SUPPLY OF THE ORGANS. 553 



and other branches from the descending aorta. From the ana- 

 tomical arrangement alone it is evident that the circulation in the 

 brain is very well protected from the possibility of being inter- 

 rupted by the accidental closure of one or more of its arteries. In 

 some animals, the dog, one can ligate both internal carotids and 

 both vertebrals without causing unconsciousness or the death of the 

 animal. In an animal under these conditions a collateral circula- 

 tion must be brought into play through the anastomoses of the 

 spinal arteries. In man, on the contrary, it is stated that ligation 

 of both carotids is dangerous or fatal. 



The Venous Supply. The venous system of the brain is peculiar, 

 especially in the matter of the venous sinuses. These large spaces 

 are contained between folds of the dura mater or, on the base of 

 the skull, between the dura mater and the bone. The channel 

 hollowed out in the bone is covered with a roof of tough, inex- 

 tensible dura mater, and indeed in some animals the basal sinuses 



Cerebrum.. 



Fig. 228. Diagram to represent the relations of the meningeal membranes of the cere- 

 brum, the position of the subarachnoidal space and of the venous sinuses. 



may in part be entirely incased in bone. The larger cerebral veins 

 open into these sinuses; the openings have no valves, but, on the 

 contrary, are kept patent and protected from closure by the struc- 

 ture of the dura mater around the orifice. The smaller veins are 

 very thin walled and free from valves. The venous blood emerges 

 from the skull in man mainly through the opening of the lateral 

 sinuses into the internal jugular vein, although there is also a 

 communication in the orbit between the cavernous sinus and the 

 ophthalmic veins through which the cranial blood may pass into 

 the system of facial veins, another communication with the venous 

 plexuses of the cord, and a number of small emissary veins. In 

 some of the lower animals the dog, for instance the main outflow 

 is into the external jugular through what is known as the superior 

 cerebral vein. A point of physiological interest is that the venous 

 sinuses and their points of emergence from the skull are by their 

 structure well protected from closure by compression. 



