DB KNOX on the Comparative Anatomy of the Eye. 73 



by the superior and inferior maxillary branches of the fifth, and 

 is probably intended to subdivide the nervous filaments, and af- 

 ford a nucleus for the origin of a greater number of nerves than 

 could otherwise have happened, had they arisen from a single 

 trunk. That this is nearly the sole use of these gangh'a, we may 

 infer, by observing that they are often wanting in animals, with- 

 out giving rise to any difference in the functions of the nerves. 

 There is no lenticular ganglion in the horse, and this explains 

 the scarcity of ciliary nerves in that animal, the little develop- 

 ment of the ciliary muscle, and his very limited powers of vi- 

 sion. The submaxillary ganglion, and that so well described by 

 MECKEL, are wanting in a great number of animals ; and the 

 latter is not unfrequently absent in man himself. There is no- 

 thing, therefore, important in these gangh'a ; nothing affecting 

 directly the functions of the parts to which the ciliary nerves are 

 distributed. The connexion of the great sympathetic nerve with 

 the lenticular ganglion is very indirect, for the supposed con- 

 necting branch cannot be traced to the ganglion, and seems ra- 

 ther to join the ophthalmic branch of the fifth, in the same way 

 as the superior maxillary is connected with the superior cervical 

 ganglion, by means of the vidian. 



The iris has been long considered an involuntary muscle, and 

 undoubtedly it is not so immediately under the power of volition 

 as many other muscles, and more particularly those of the extre- 

 mities ; but the assertion that the iris is altogether an involun- 

 tary muscle, and dependent for its motions on the stimulus of 

 light acting on the retina, may, I think, be disproved by the 

 simplest experiments. The iris dilates or contracts according 

 as the attention is more or less energetically fixed on an object, 

 though the quantity of light transmitted to the retina remain pre- 

 cisely the same. The cat and owl have the most complete command 

 over the motions of their iris, a fact well known to every one ; 



VOL. x. P. i. K 



