496 



NERVOUS SYSTEM 



being its relations to the movements of the iris, concerning which the 

 results of experiments are somewhat contradictory. 



Physiological Anatomy. The apparent origin of the third nerve is 

 from the inner edge of the crus cerebri, directly in front of the pons 

 Varolii and midway between the pons and the corpora albicantia. It 

 presents here eight or ten filaments, of nearly equal size, which soon 

 unite into a single rounded trunk. 



The deep origin of the nerve has been studied by means of dissec- 

 tions of the encephalon fresh and hardened by different liquids. From 



the groove by which they emerge from 

 the encephalon, the fibres spread out 

 in a fan-shape, the middle filaments 

 passing inward, the anterior, inward 

 and forward, and the posterior, in- 

 ward and backward. It is probable 

 that the middle filaments pass to the 

 median line and decussate with corre- 

 sponding fibres from the opposite 

 side. The anterior filaments . pass 

 forward and are lost in the optic 

 thalamus. The posterior filaments on 

 either side pass backward to a gray 

 nucleus beneath the aqueduct of Syl- 

 vius and here decussate with fibres 

 from the opposite side. This decus- 



Fig 



118. Distribution of the motor oculi 

 communis (Hirschfeld). 



, trunk of the motor oculi communis ; 2,su- 



oculi externus ; 9, filaments of the motor oculi 

 externus anastomosing with the sympathetic; 

 10, ciliary nerves. 



perior branch ; ^ filaments which this'branch Sation of the fibres of Origin of the 



sends to the superior rectus and the levator third nerves J s important in COnttCC- 

 palpebri superwrts ; 4, branch to the internal 



rectus; 5, branch to the inferior rectus; tion with the harmony of action of 



6, branch to the inferior oblique muscle; f i _, ., miieplpe n f the PVPS 



7, branch to the lenticular ganglion ; 8, motor [ tne C y eS 



on the two sides. 



The third nerve, as it passes into 



the orbit by the sphenoidal fissure, 



divides into two branches. The superior, which is the smaller, passes 

 to the superior rectus muscle of the eye and certain of its filaments are 

 continued to the levator palpebrae superioris. The inferior division 

 breaks up into three branches. The internal branch passes to the 

 internal rectus muscle ; the inferior branch, to the inferior rectus ; the 

 external branch, the largest of the three, is distributed to the inferior 

 oblique muscle, and in its course it sends a short and thick filament to 

 the lenticular, or ophthalmic ganglion of the sympathetic. It is this 

 branch which is supposed, through the short ciliary nerves passing from 

 the lenticular ganglion, to furnish motor filaments to the ciliary muscle and 



