•180 



VEIXS OF THE HEAD AND NECK. 



(//) The middle thi/roid vein, likewise derived from the thyi'oid body, is placed 

 lower than the superior thyroid. 



VENOUS CIRCULATION WITHIN THE CRANIUM. 



The part of the venous system contained within the skull consists of 

 veins properly so called, and of certain channels called sinuses, which 

 receive the blood from those veins, and conduct it to the internal 

 jugular veins. The sinuses alluded to are spaces left between the 

 layers of the dura mater, the fibrous covei'ing of the brain. 



CEREBRAL VEINS. 



The veins of the brain are divisible into those which ramify upon its 

 surface, and those which are placed within its ventricles. 



Fig. 305. 



Fig. 305. — Internal View of the Bask of thk Skull, showing the Sinuses op the 

 Dura Mater, &c. (A. T.) { 



The sinuses of the dura mater have been opened, a small portion of the roof of the 

 orbit has been removed posteriorly on the left side, and the dura mater has been dissected 

 so as to bring into view the arteries at the base of the skull, the venous sinuses and the 

 trunks of the cranial nerves. 



I., the olfactory bulb ; II., the optic nerves ; III., placed on the pituitary body, 

 indicates the third nerve ; IV., the trochlear nerve ; V., placed oi^posite to the middle 

 of the three divisions of the fifth nerve ; VI., the sixth nerve ; VII., the facial and 

 auditory nerves ; VIII. , jjlaced opposite to the three portions of the eighth pair ; IX. , the 



