610 



NERVOUS SYSTEM. 



Physiological Anatomy.-!*** all of the cranial nerves, this has an apparent origin, 

 where it separates from the encephalon, 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 third nerve is from the inner edge of the crus cerebn, direct- 

 ly in front of the pons Varolii, 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 dissections 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, 

 inward and backward. It is probable that the 

 middle filaments pass to the median line and de- 

 cussate with corresponding fibres from the oppo- 

 site side. The anterior filaments pass forward 

 and are lost in the optic thalamus. The poste- 

 rior filaments on either side 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. 



The distribution of the third nerve is very 

 simple. As it passes into the orbit by the sphe- 

 noidal fissure, it divides into two branches. The 

 superior, which is the smaller, passes to the su- 

 perior rectus muscle of the eye, and certain of 

 FIG. 196. Distribution o the motor oculi com- its filaments are continued to the levator palpe- 



b s p eri ris - The inferior division breaks U P 



into three branches. The internal branch pass- 



, , . ,, . 



es to the internal rectus muscle ; the interior 



branch, to the inferior rectus; the external 



branch > the iar f st of the three> is d : stributed to 



H extemus anastomosing with the sympathet- 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 the motor influ- 

 ence to the iris. In its course, this nerve receives a few very delicate filaments from the 

 cavernous plexus of the sympathetic and a branch from the ophthalmic division of 

 the trifacial. 



Properties and Functions of the Motor Oculi Communis. Irritation applied to the 

 root of the third nerve in a living animal produces contraction of the muscles to which 

 it is distributed, but no pain. If the irritation, however, be applied a little farther on, 

 in the course of the nerve, there are evidences of sensibility, which is readily explained 

 by its communications with the ophthalmic branch of the trifacial. At its root, there- 

 fore, this nerve is exclusively motor, and its functions are connected entirely with the 

 action of muscles. 



Most of the important facts bearing upon the functions of the motor oculi are clearly 

 demonstrable by dividing the nerve in a living animal and are illustrated by cases of its 

 paralysis in the human subject. All physiologists who have divided the nerve in living 

 animals are agreed with regard to the phenomena following its section, which depend 

 upon paralysis of the voluntary muscles. These phenomena are as follows : 



. rior branch ; ^jUaments which this branch 

 sends to the superior rectus and the levator 

 paipebri superioris ; 4, branch to the inter- 



