THE OCULOMOTORIUS. 523 



Decussation of the Oculomotor ius Nerve. According to the observa- 

 tions of Meynert, a decussation takes place between the oculomotorius 

 nucleus and the opposite side of the brain, by means of fibres emerging 

 from the raphe upon the median line, near which the nucleus is situated. 

 These fibres come originally from the corpus striatuin, thence running 

 backward along the inner border of the crura cerebri, into the longitu^ 

 clinal lamina forming the raphe between them. Underneath the aque^ 

 duct of Sylvius they decussate with each other at acute angles, those 

 from the right corpus striatnm passing to the nucleus of the left side, 

 and vice versa. Each oculomotorius nerve is therefore in connection 

 with the opposite side of the brain, not by means of its own fibres, but 

 through the intervention of its nucleus and the fibres which pass thence, 

 through the raphe, toward the opposite corpus striatum. 



Physiological Properties of the Oculomotorius Nerve. The oculomo- 

 torius is in itself an exclusively motor nerve, and has been found by 

 Longet, when examined in the living animal, near its point of emergence 

 from the crus cerebri, to be insensible to mechanical irritation ; but at 

 some distance farther forward, after receiving its branches of commu- 

 nication from the fifth pair, it exhibits a certain degree of sensibility. 

 Its excitability, on the contrary, is very manifest ; and its irritation 

 within the cranial cavity, even after it has been separated from its con- 

 nection with the brain, causes convulsive action in the muscles of the 

 eyeball. 



The physiological function of this nerve is distinctly shown by the 

 nature of the paralysis following its section either before or after its 

 entrance into the orbit. These results are for the most part very sim- 

 ple and well marked, and are established by the uniform testimony of 

 various observers. They consist of the paralysis of the five muscles to 

 which the nerve is distributed, and induce, consequently 



1. External strabismus, from continued action of the external straight 

 muscle of the eyeball, which is no longer controlled by that of the 

 internal. 



2. General immobility of the eyeball, owing to the abolition of its 

 natural upward, downward, lateral, and rotatory movements. For 

 although two of the muscles of the eyeball, namely, the external rectus 

 and the superior oblique, remain unparalyzed ; yet, as they are no 

 longer antagonized by the remainder, they can only produce a perma- 

 nent deviation of the eyeball, but no alternate movement in opposite 

 directions. In most of the lower animals there is also an unusual 

 prominence of the eyeball, owing to the relaxed condition of the muscles 

 which serve for retraction. 



3. Drooping of the upper eyelid. In the ordinary action of opening 

 the eye, it is the upper eyelid alone which moves, being raised so as to 

 uncover the cornea and pupil, by the contraction of the levator palpebrse 

 superioris. As this muscle is animated by a nervous branch coming 

 from the oculomotorius, it is paralyzed by section of this nerve at the 

 same time with the muscles moving the eyeball. The consequence is 



