MOTOR OCULI COMMUNIS. 



541 



sound, for the auditory nerves. The proper transmission of these impres- 

 sions, however, involves the action of accessory parts, more or less complex ; 

 and the properties of these nerves will be fully considered in connection with 



the physiology of the special senses. 



MOTOR OCULI COMMUNIS (THIRD NERVE). 



The third cranial nerve is the most important of the motor nerves dis- 

 ;ributed to the muscles of the eyeball. Its physiology is readily understood 

 in connection with its distribution, the only point at all obscure being its re- 

 lations to the movements of the iris, upon 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, mid- 

 way 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 dissections of the en- 

 cephalon 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 mid- 

 dle filaments passing inward, the anterior, 

 inward and forward, and the posterior, 

 inward and backward. It is probable 

 that the middle filaments pass to the me- 

 dian line and decussate with correspond- 

 ing 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 

 Sylvius and here decussate with fibres 

 from the opposite side, This decussation 

 of the fibres of origin of the third nerves 

 is important in connection with the har- FIG. 194. Distribution of the motor 



communis (Hirschfeld). 



mony of action of the iris and the mus- i, trunk of the motor oc*Ueomm*nii ;*,* 



cles of the eyes upon the two sides. P erim '- bran - ch - ; * fi laments whi . ch 



The distribution of the third nerve is 

 very simple. As it passes into the orbit, 

 by the sphenoidal fissure, it divides into 

 two branches. The superior, which is the 

 smaller, passes to the superior rectus mus- 

 cle of the eye, and certain of its filaments are continued to the levator palpe- 

 bras 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 distribu- 

 ted to the inferior oblique muscle, and in its course, it sends a short and 

 36 



branch sends to the superior rectus and 

 the levator palpebri superioris ; 4, branch 

 to the internal rectus ; 5, branch to the 

 inferior rectus ; 6, branch to the inferior 

 oblique muscle ; 7, branch to the lenticular 

 ganglion ; 8, motor oculi externus ; 9, fila- 

 ments of the motor oculi externus anasto- 

 mosing with the sympathetic ; 10, ciliary 

 nerves. 



