102 INNERVATION. [CHAP. XIX. 



the sympathetic ; and, as has been asserted by a few anatomists, 

 with the sixth nerve. 



Function of the Third Nerve. Proceeding according to the 

 method indicated at page 303, vol. i., we deduce the function of 

 this nerve, from its anatomy in man, from its anatomy in animals, 

 from experiments, and from pathological observation. 



From its anatomy in man, we judge this nerve to be a motor 

 nerve, for it is distributed entirely to muscles. These muscles are, 

 in addition to the elevator of the upper eyelid, those upon which 

 the principal movements of the eyeball depend, indeed, all the 

 muscles of that organ, except the superior oblique and the external 

 rectus. In addition to these there can be no doubt that the third 

 nerve sends filaments to the muscular apparatus within the eye by 

 the short root of the ophthalmic ganglion, which, after passing 

 through that ganglion, escapes from it under the form of ciliary 

 nerves, which may be traced to the ciliary muscle and to the iris. 

 It is, therefore, the principal motor nerve of the eyeball, regulating 

 all the movements of that organ, excepting those which depend 

 on the external rectus and superior oblique muscles ; and it also 

 probably excites the movements of the iris and of the other muscular 

 fibres within the eye. Upon this latter point, however, anatomy 

 speaks less positively, from the fact that other nerves are distribu- 

 ted to the iris besides those derived from the third. 



The distribution of the third nerve, in the inferior animals, 

 leads to the same conclusion respecting its function, as that which 

 we have deduced from human anatomy. In all the mammalia it is 

 distributed to the same muscles of the eyeball as in man, and to the 

 iris. In birds a similar arrangement exists, and in some species of 

 this class we observe a remarkable development of that branch 

 which is distributed to the iris in direct relation with the muscular 

 activity of that structure. In the falcons and eagles, according to 

 Desmoulins, the third nerve is absolutely as large as in man, and 

 this great size is due to the development of the branch which is 

 distributed to the iris and ciliary muscle. 



Experiments on this nerver are difficult of execution, and their 

 results are therefore not free from complication ; but such as have 

 been obtained entirely confirm the view of its function which ana- 

 tomy suggests. These results, as derived from the experiments of 

 Rumbold and Fowler, of Mayo, Valentin, and others, may be 

 summed up as follows : 



The application of the galvanic current to the nerve causes a con- 

 vulsive contraction of the principal muscles of the globe, and of the 



