CORPORA QUADRIGEMINA 



'11 



the lateral groove of the mesencephalon under cover of the pulvinar. Its long axis 

 is directed transversely. A band of white fibres is prolonged from its outer end 

 into the optic tract. This passes as the hindermost layer of fibres in the optic 

 commissure into the corresponding bundle of the opposite side. It is therefore 

 commissural in nature. It does not contribute any fibres to the optic nerve. 



Tlie corpus geniculatum externum is a slight elevation at the outer side of 

 the pulvinar. It is directly continuous with the optic tract. 



The remainder of the mesencephalon is formed b}' the crura cerebri, each of 

 which is divided into three distinct parts easily recognised in transverse section (fig. 



Fig. 424. — Deep Origin of the TiiiEn Xerve. (After Krause 









AQUEDUCT OF SYLVIUS 



DESCENDING 

 ROOT Of 

 TRIGEMINAL 

 \'^ER VE 



CLEUS OF 

 THIRD NERVE 



POSTERIOR 

 LONGITUDINA L 

 BUNDLE 



SUBSTANTIA 

 NIGRA 



RAPHE 



THIRD NER VE 



r^k 



•4t 





,t*'. ',' 



%. 



CRUST A 



mm 



424). These are (1) a greyish upper portion, the tegmen ; (2) a band of almost 

 black substance, the substantia nigra ; and (8) a lower white portion, the crusta, 

 which appears at the base of the brain. Tlie tegmen is also marked of¥ superticially 

 from the crusta b}' two grooves, the oculo-motor and the lateral groove, wliich 

 may be seen in section to correspond with the inner and outer extremities of the 

 substantia nigra. The oculo-motor groove derives its name from the third or oculo- 

 motor nerve, the filaments of which reach the surface along this furrow. The 

 lateral groove of the mesencephalon is best marked near the upper border of the 

 pons, where it is bounded below by the superficial fibres of the crusta; and above 



