THE HEAD AND NECK. 135 



muscle, and joins the gustatory nerve. It is distributed to the submaxillary and 

 sublingual glands and to the dorsum of the tongue. 



Dissection of the Seventh Nerve in the Petrosa and of the Tympanum. Locate 



the following anatomical structures : 







1. The superior border of the petrosa. 



2. Anterior and posterior surfaces of the petrosa. 



3. The trigeminal notch near apex of petrosa. 



4. The cave of Meckel on anterior surface of petrosa. 



5. The horizontal part of the carotid canal and its artery. 



6. The great petrosal nerve in the hiatus Fallopii. 



7. The small petrosal nerve in its small canal. 



8. The middle meningeal artery and foramen spinosum. 



9. The tegmen tympani or tympanic roof. 



10. The internal auditory meatus on posterior surface of the petrosa. 



With a small chisel and mallet gently remove the tegmen in such a manner as 

 not to destroy the delicate petrosal nerves on the anterior surface of the petrosa, 

 and then you can look down into the tympanum, or middle ear, the roof now 

 having been removed (tegmen). Now draw a line from the internal auditory 

 meatus to where the great petrosal nerve is seen emerging. This line will locate 

 the course of the seventh nerve in its canal to the tympanum. Now begin at the 

 internal end of the line just drawn and chip off the bone until you have completely 

 exposed the seventh nerve to the tympanum, where you will see the geniculate 

 ganglion giving off petrosal nerves. 



The tympanum may be represented as a box with an inner and an outer 

 wall, a roof and a floor, an anterior and a posterior end. The seventh nerve 

 approaches the tympanum, at a point corresponding to the junction between 

 the roof and inner wall, near the front end of the box. The seventh nerve 

 having gained this point, now passes backward in the angle between the roof 

 and inner wall to the corner of the box between the inner wall and posterior 

 end of the box. The nerve here makes a gentle turn and passes down this cor- 

 ner to the floor of the box, and subsequently emerges from the base of the skull 

 through the stylo-mastoid foramen. You may see the geniculate ganglion of the 

 nerve just where the nerve is making the bend to enter the tympanum. Now 

 locate the nerves as described previously, and consult figure 90. 



BLOOD-SUPPLY TO THE BRAIN. 



Examine the base of the brain. You notice very distinctly the arachnoid 

 membrane quite appreciably separate from the pia. In other words, here you 

 see the subarachnoid space. Now remove carefully the arachnoid and dissect 

 the circle of Willis. You will note the following : 



1 . The vertebral arteries unite to form the basilar artery. 



2. The basilar divides into the two posterior cerebral arteries. 



3. The internal carotid artery gives off three large arteries (Fig. 91) : 

 (i) The anterior cerebral, in the great longitudinal fissure ; (2) the middle cerebral, 

 in the fissure of Sylvius on the central lobe ; (3) the posterior communicating, to 

 the posterior cerebral. 



The arachnoid membrane, it will be observed, stretches across from frontal 

 to temporo-sphenoidal lobe, so as almost to obscure the fissure of Sylvius. Cut 

 this membrane with the scissors, gently part the lobes, and you will see the 

 island of Reil, on which is the middle cerebral artery with its vein. 



The anterior communicating artery is very short, one-eighth of an inch 

 being an average length in a number of cases. You will find it extending from 

 one anterior cerebral artery to the other. 



