166 Mammalia. 



of nerves {i.e. th.e chief Motor nerves of the muscles of the 

 eyeball), and are crossed just before entering the hemispheres 

 by the optic tracts to which they are adherent, are the Crura 

 Cerebri. 



Posterior to the Corpus Albicans, bounded on either side 

 by the Crura Cerebri, and limited posteriorly by the Pons 

 Varolii, is the Posterior perforated space, which forms the back 

 part* of the floor of the third ventricle, and is perforated by 

 numerous small orifices for the passage of blood vessels to the 

 Thalami optici. 



Winding round the outer side of either Crus from above, 

 having origin in the upper part of the Yalve of Yieussens, 

 immediately behind the Testes, and above the Pons Varolii, 

 will be seen the Fourth Pair of Nerves. They are also called the 

 Trochlear'^ nerves, and supply the orbital surface of the su- 

 perior oblique muscle of the Eye. 



The broad transverse band of white fibres which forms the 

 great transverse cerebellar commissure, and whose extremities, 

 contracted into a thick rounded form, constitute the middle 

 crura, or peduncles, of the Cerebellum, is the Pons Varolii. Its 

 under surface presents a groove in the median line, in which 

 was lodged the basilar artery ; its upper surface forms part of 

 the floor of the fourth ventricle ; and from its sides, the Fifth 

 Pair of Nerves f nerv. trigeminus, or Trifacial, take their super- 

 ficial origin. 



The Trifacial^ is the largest cranial nerve, and somewhat 

 resembles a spinal nerve in its origin by two roots, one motor, 

 the other sensory, and in the existence of a ganglion (Gasse- 

 rian) on its sensory root : it has three great divisions. The 



* Cf. supra, Tuber Cinereum. 



t The Superior Oblique Muscle in Man passes through a fibrous loop at the 

 inner angle of the orbit before reaching the eyeball, thus as it were, passing 

 through a pulley or trochlea, (Gr. vgax,i7^ia) whence the name given to the nerve 

 supplying it. 



X Also called Trigeminal. 



