THE BRAIN. ORIGIN OF NERVES. 1051 



arachnoid membrane and pia mater are stripped off, except from the angle be- 

 tween the cerebellum and medulla oblongata, the principal parts of the brain 

 visible from the base are to be examined. These are : the fissure of 

 Sylvius separating the anterior from the middle lobe, and contained in it 

 the hidden convolutions or island of Reil ; at the entrance of the fissure the 

 locus perforatus anticus, and terminating in it the inferior part of the trans- 

 verse fissure of the cerebrum ; also, the crura cerebri emerging from before 

 the pons Varolii, the anterior extremity of the corpus callosum lying in the 

 bottom of the great longitudinal fissure, and below it, proceeding backwards 

 in the middle line, the lamina cinerea, the optic commissure, the tuber 

 cinereum, the infundibulum, the corpora albicantia, and the locus per- 

 foratus posticus (p. 536). The principal objects to be noted on the medulla 

 oblongata are the anterior pyramids with their decussation and the olivary 

 bodies on the front, and the restiform bodies on its lateral aspect ; pos- 

 teriorly are the posterior pjramids, and the calamus scriptorius, and its 

 prolongation downwards into the minute remains of the central canal of the 

 spinal cord (p. 514). The fourth ventricle, situated between the medulla 

 oblongata and cerebellum, is now brought into view, and at its sides will be 

 observed the fringes of pia mater called choroid plexus of the fourth ven- 

 tricle, the two small lobules of the cerebellum at the sides of the medulla 

 oblongata named the flocculi or subpeduncular lobes, and behind them the 

 amygdalae ; while above the medulla are the parts belonging to the middle 

 lobe of the cerebellum, afterwards more fully noticed (p. 521). 



The origins of the cranial nerves may next be examined. The first pair 

 or olfactory tracts and bulbs are seen on the anterior lobes, and should be 

 traced back to the white strise by which they arise at the inside of the 

 fissure of Sylvius ; the second and fourth nerves are seen passing round the 

 crura cerebri, the optic tracts from the corpora quadrigemina, optic thalami, 

 and corpora geniculata, the fourth nerve from the valve of Vieussens ; the 

 third pair lying close together on the inner aspects of the crura cerebri ; 

 the fifth pair emerging by two roots from the front of the pons Varolii ; the 

 sixth in front of the anterior pyramids ; the seventh nerve in two parts, 

 the portio dura and portio mollis, in the angle between the medulla ob- 

 longata, pous Varolii, and cerebellum ; the eighth pair in three parts, the 

 glosso-pharyngeal, vagus or pneumo-gastric, and spinal accessory, in front of 

 the restiform body ; the ninth pair in front of the olivary body ; and the 

 suboccipital (or first cervical nerve of some authors) close below the ninth 

 (p. 583). The cerebellum is to be separated from the structures to which 

 it is attached by division of its superior, middle and inferior crura. The 

 general disposition of its convolutions and the superior vermiform process 

 will be noted, as also the parts entering into the formation of the inferior 

 vermiform process lying in the vallecula beneath, viz. , the pyramid, uvula, 

 and laminated tubercle, together with the posterior velum. Sections of the 

 cerebellum are to be made to exhibit the arbor vitae and the grey centre 

 known as the corpus dentatum or rhomboideum. In conclusion, sections 

 may be made of the pons Varolii to show its transverse and longitudinal 

 fibres, of the medulla oblongata to show the olivary nucleus or corpus den- 

 tatum, and of the crura cerebri to show the locus uiger. 



The venous sinuses, arteries, and nerves in the base of the skull, ought 

 now to be examined, if there be time, before the subject is turned on its 

 back. The superior longitudinal sinus, the inferior longitudinal and the 

 straight sinus (with the veins of Galen entering it), and the poste- 

 rior occipital sinus, are to be traced to the torcular Herophili ; and the 



