XXXX.] THE BEAIN. 313 



its posterior triangular portion is the calamus 

 scriptorius; note the opening of the central 

 canal of the spinal cord into this; note also 

 at the anterior end of the fourth ventricle the 

 aqueduct of Sylvius or iter underneath the 

 corpora quadrigemina ; it runs from the fourth 

 to the third ventricle (cp. § 19). 



8. Trace the chief blood-vessels running in the pia 

 mater of the under surface of the brain, tear- 

 ing away the pia mater where necessary but 

 being careful not to tear away at the same time 

 the nerve-roots. At the upper part of the 

 medulla will be seen two arteries. These are 

 the vertebral arteries, which having given 

 off recurrent branches along the anterior fissure 

 of the medulla, curve round to unite in the 

 median line; the vessel formed by their union 

 is called the basilar artery, and runs forwards 

 in the median line of the pons Varolii. At 

 the front edge of this it divides into the two 

 posterior cerebral arteries, each of which 

 .running obliquely forward passes to the median 

 side of the third nerve as it springs from the 

 cms cerebri. 



Just beyond this each posterior cerebral divides 

 into two branches. One of these runs back- 

 ward; the other proceeding forwards, is, a little 

 behind the optic commissure, joined by the 

 internal carotid artery. Each arterial trunk 

 so formed passes round the optic commissure, 

 and divides into the middle cerebral artery 



