12 APPLIED ANATOMY. 



It is of importance in operating for infection arising from middle-ear disease. Minute 

 veins run from the antrum into the lateral sinus. 



Suprameatal Triangle. This triangle, so named by Macewen, is formed 

 above by the posterior root of the zygoma, anteriorly by the bony posterior wall of 

 the external auditory meatus and posteriorly by a line from the floor of the meatus 

 passing upward and backward to meet the first line. The mastoid antrum is reached 

 by operating through this triangle (see section on Ear) . 



Cerebral Venous Sinuses. The fibrous membrane which lines the inte- 

 rior of the skull is composed of two layers which are in most places intimately 

 united, forming one single membrane known as the dura mater. The outer 

 layer is applied to the bone, while the inner layer covers the brain. In certain 

 places these two layers separate to form channels in which venous blood flows; 

 these channels are called sinuses. In certain other places these layers separate and 

 enclose some special structure, as the Gasserian ganglion. 



The cerebral sinuses of most importance are the superior longitudinal, the lateral 

 or transverse, and the cavernous. 



The siiperior longitudinal sinus runs in the median line from the foramen caecum 

 in the ethmoid bone in front, to the torcular Herophili behind. As it passes back- 



FIG. 18. Posterior view of the skull, showing the relation of the superior longitudinal sinus and torcular Herophili 

 to the median line and external occipital protuberance. 



ward it inclines more to the right side, so that at the torcular Herophili the left side 

 of the sinus is about in the median line. This sinus receives the veins from the 

 cortex of the brain and also some from the diploe of the bones above it. A vein 

 pierces the upper posterior angle of each parietal bone and forms a communication 

 between the superficial veins of the scalp outside and the superior longitudinal sinus 

 within. The deviation of the superior longitudinal sinus toward the right, as it 

 proceeds posteriorly, is to be borne in mind in operating in this region, as one can 

 approach the median line nearer on the left side posteriorly than the right, without 

 wounding it. In the parietal region the Pacchionian bodies are surrounded by 

 extensions from the longitudinal sinus and free hemorrhage will ensue if the bone is 

 removed too close to the median line. 



The torcular Herophili, or confluence of the sinuses, does not correspond exactly 

 to the external occipital protuberance or inion on the exterior of the skull. It is 

 a little above and to the right of it. This torcular Herophili is formed by the meet- 

 ing of the longitudinal sinus from above, the lateral, or transverse sinuses from 

 the sides, the straight sinus from in front and the occipital sinus from below. 



The lateral or transverse sinuses, of which there are two, pass from the torcular 

 Herophili toward each side in the tentorium between the cerebrum and cerebellum, 

 following the superior curved line of the occiput until just above the upper posterior 

 portion of the mastoid process. They then bend downward to within a centimetre of 



