n6 HEAD AND NECK 



jacent cerebellum. Next turn the head so as to bring its left 

 side uppermost, and treat the tentorium on that side in the same 

 manner. Now let the brain fall well backwards ; then the pons 

 and medulla will be drawn away from the anterior wall of the 

 posterior fossa of the skull, and the nerves in the posterior fossa 

 will come into view. They are the two parts of the trigeminal 

 nerve (fifth cerebral nerve), perforating the dura mater near 

 the apex of the petrous portion of the temporal bone ; the 

 abducent nerve (sixth cerebral nerve), piercing the dura mater 

 behind the dorsum sellae of the sphenoid bone ; the facial nerve 

 and the acoustic nerve, disappearing into the internal acoustic 

 meatus ; the glossopharyngeal, the vagus, and the accessory 

 nerves, leaving the skuli through the jugular foramen; and 

 the two slips of the hypoglossal nerve, piercing the dura mater 

 over the hypoglossal canal (O.T. anterior condyloid foramen). 

 Each in turn will be displayed upon each side. They must be 

 divided in the order mentioned, except that, in the case of the 

 nerves passing out of the cranium through the jugular foramina, 

 the dissector should endeavour to leave the accessory of the 

 right side intact within the cranium, by dividing its roots of 

 origin from the medulla oblongata, whilst on the other side he 

 should remove it with the brain. The accessory nerve is readily 

 recognised because its spinal part ascends from the vertebral 

 canal into the cranial cavity through the foramen magnum. 

 Now, thrust the knife into the vertebral canal, and divide the 

 medulla spinalis and the vertebral arteries, as they turn forwards 

 upon the upper part of the medulla spinalis (O.T. spinal cord) ; 

 then sever the accessory nerve of the left side, and the roots of 

 the first pair of spinal nerves. When that has been done let 

 the head fall well downwards, gently dislodge the medulla 

 oblongata and cerebellum, and the whole brain can be removed. 

 The vena cerebri magna (Galen), as it passes from the interior 

 of the brain to enter the straight sinus, is ruptured as the brain 

 is removed. The dissector should now return to p. 112, and 

 should study the positions and relations of the cranial blood 

 sinuses. 



Meningeal Veins. In addition to the named blood sinuses, venous 

 channels accompany the meningeal arteries, and more particularly the 

 trunks and branches of the middle meningeal artery. The meningeal veins 

 are of wider calibre than the corresponding arteries, and lie external to them, 

 in the grooves on the inner surfaces of the cranial bones. When the 

 arteries are distended they compress the middle parts of the veins and 

 drive the blood into their anterior and posterior margins. When that 

 occurs each artery appears to be accompanied by two veins, a circumstance 

 which is probably responsible for the statement that some of the meningeal 

 arteries have venae comites. 



Emissaria. Emissary veins are blood channels which 

 connect the sinuses of the dura mater with the veins which 

 lie outside the cranium. They are: (i) Emissary veins con- 

 nected with the superior sagittal sinus (a) from the anterior 

 extremity of the sinus an emissary vein passes through 

 the foramen caecum ; this vein divides below, and either 



