456 THE NERVOUS SYSTEM. 



twigs of sensitive fibres from the trigeminus. On entering the orbit, 

 it divides into several branches, supplying the superior, inferior, and 

 internal straight muscles of the eyeball, the inferior oblique, and the 

 levator palpebrae superioris. The oculomotorius is accordingly con- 

 cerned both in the vertical and lateral movements of the eyeball, and 

 in those of rotation ; while of the two other muscular nerves of this 

 organ, the abducens is connected only with the movement of abduc- 

 tion, the patheticus only with that of rotation. 



Decussation of the Oculomotorius Nerve. According to Meynert, 

 a decussation takes place between the oculomotorius nucleus and the 

 opposite side of the brain by fibres crossing the median line in the 

 raphe, near which the nucleus is situated. These fibres come originally 

 from the corpus striatum, thence run backward along the inner border 

 of the crus cerebri, and into the longitudinal lamina forming the raphe. 

 Underneath the aqueduct of Sylvius they decussate at an acute angle, 

 those from the right corpus striatum passing to the nucleus of the left 

 side, and vice versa. Each oculomotorius nerve is therefore in con- 

 nection 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, to the opposite corpus striatum. 



Physiological Properties of the Oculomotorius Nerve. The oculo- 

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

 Longet, near its point of emergence from the crus cerebri, insensible 

 to mechanical irritation ; but at some distance farther forward, after 

 receiving its branches of communication from the trigeminus, it exhibits 

 a certain degree of sensibility. Its excitability, on the contrary, is 

 very manifest ; and its irritation within the cranial cavity, even after 

 its separation from the brain, causes convulsive action in the muscles 

 of the eyeball. 



The physiological function of this nerve is shown by the paralysis 

 following its section either before or after its entrance into the orbit. 

 These results are for the most part simple and well marked, and are 

 established by the uniform testimony of various observers. They con- 

 sist 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 antagonized by the 

 internal. 



2. Immobility of the eyeball, owing to the abolition of its 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 permanent deviation of the 

 eyeball, but no alternate movement in different directions. In most of 

 the lower animals there is also an unusual prominence of the eyeball, 

 owing to paralysis of the retractor muscles. 



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



