TROCHLEAR AXD TRIGEMLXAL XERVES 



749 



the substance of which they decussate with their fellows of the opposite side and 

 become collected into a single bundle (fig. 444). This Vmndle emerges as a slender 

 rounded fasciculus (superficial origin) from the valve of Vieussens, close to the 

 frenulum veli, immediately Vjelow the testes, and mnds round the crus cerebri to 

 ap])ear at the base of the brain near the anterior margin of the pons. The right 

 nerve, therefore, arises from the left nucleus and vice versa. 



The fourth nerve ])ierces the dura mater at a point a little behind and external 

 to the posterior clinoid process of the sphenoid bcjne in the posterior fossa of the 

 skull. It runs forwards in the outer wall of the cavernous sinus, being placed 

 between the third nerve, which is above and internal to it, and the ophthalmic 



Fig. 444. — Sections through the Origin of the Fourth Nerve. (Stilling.) 

 (The upper fij^nre is an oblique section, the lower is a coronal section.) 



AQUEDUCT OF SYLVIUS 



yUCLEUS OF FOURTH 

 NER VE 



POSTERIOR LONGI- 

 TUDINAL BUNDLE 



AQUEDUCT OF SYLVIUS 



SUPERIOR CEREBELLAR 

 PEDUNCLE 



division of the fifth nerve, which is situated below and to its outer side. As it 

 ajujroaches the sphenoidal fissure it bends upwards, crosses on the outer side of the 

 tliird nerve, and passes through the innermost part of the sphenoidal fissure. As 

 it passes through the fissure it is placed above and internal to the frontal nerve. 

 It then passes inwards above the origin of the levator palpebrae superioris muscle, 

 and pierces the orbital surface of the superior oblique muscle, in which it ends. 



The fourth nerve is the smallest of the cranial nerves. It is remarkable for the 

 length of its intracranial course and for its mode of decussation. \\'liilst in the 

 wall of the cavernous sinus it is connected with the ophthalmic branch of the fifth 

 nerve and with the cavernous plexus of the sympathetic. 



FIFTH OR TRIGEMINAL NERVE 



The fifth or trigeminal nerve consists of two parts, a large sensory root (portio 

 major) and a small motor root (portio minor). The portio major passes into a 

 ganglion (Gasserian ganglion) which has been compared to the ganglion on the 

 posterior root of a spinal nerve, which it probably resembles in exercising a tropliic 

 influence on the nerve-fibres, though this has not been experimentally proved. The 

 'superficial origin' of the nerve is from the side of the pons near its upper border. 

 The deep origin of the portio major is chiefly l)y the ascending root, a long tract 

 of fibres which runs for a con.siderable distance within the cerebro-spinal axis. The 

 portio major has, however, in addition to this, several supplementary origins. The 

 portio minor arises principally from a nucleus which is embedded in the grey 



