MOTOR OCULI COMMUNIS. 127 



the movements of the iris, upon which the results of experi- 

 ments are somewhat contradictory. As a preface to the 

 study of the functions of this nerve, it is necessary to de- 

 scribe its anatomical relations. 



Physiological Anatomy. Like all of the cranial nerves, 

 this has an apparent origin, where it separates from the en- 

 cephalon, and a deep origin, which is the last point to which 

 its fibres can be traced in the substance of the brain ; but 

 the origin has not the physiological importance attached to 

 its ultimate distribution. 



The apparent origin of the nerve is from the inner edge 

 of the cms .cerebri, directly in front of the pons Yarolii, 

 midway between the pons and the corpora albicantia. It 

 presents here from eight to ten filaments, of nearly equal 

 size, which soon unite into a single, rounded trunk. 



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

 tions of the encephalon fresh and hardened by different 

 liquids. Yulpian, who has made a great number of very 

 careful dissections of these nerves, has been able to follow 

 the fibres from their apparent origin into the brain-substance 

 as tar as the median line.* 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, in- 

 ward and forward, and the posterior, inward and backward. 

 As the result of his observations, Yulpian concludes that 

 the middle filaments pass to the median line, and decussate 

 with corresponding fibres from the opposite side. The ante- 

 rior filaments pass forward and are lost in the optic thala- 

 mus. The posterior filaments pass backward, and decussate 

 beneath the aqueduct of Sylvius. This apparent decussation 

 of the fibres of origin of the third nerves is important in 

 connection with the harmony of action of the muscles of 

 the eyes and the iris upon the two sides. 



1 VULPIAN, Exsai sur Vorigine de plusieurs paires des nerfs craniens, These, 

 Paris, 1853, p. 10, et seq. 

 109 



