On the accommodation of the Eye to distances. 221 



This simple mechanism, will not succeed in adjusting a lens that 

 is not a sphere, without altering the direction of the rays of light. 

 Another and a more complicated structure exists in the amphibia 

 that I have examined, and in animals that live exclusively in air. 

 The ciliary processes arranged in a radiated manner around the lens 

 are very vascular. When wounded, they efRise so much blood that 

 they are cautiously avoided by the operator. Each process is like a 

 folded leaf having its sides of a triangular form.* In the eye of an 

 animal recently killed they adhere firmly to that portion of the hyaloid 

 membrane that forms the anterior wall of the canal of Petit, and can 

 only be seperated from it by rupture. The ruptured portion forms 

 the halo signatus. At their base upon the inner surface of the choroid 

 coat there is a range of muscular fibres. In the sheep the fibres of 

 the upper portion run transversely to the cihary processes. Those of 

 the lower portion run parallel to them. The transverse fibres leave 

 a distinct impression upon the hyaloid membrane when they are sep- 

 arated from it. Petit's canal is broader here than it is below. 



When Petit's canal is inflated without removing the support of the 

 vitreous humor the crystalline advances. When the inflation is dis- 

 continued it resumes its place. Petit's canal thus appears to be formed 

 for limiting its motion and maintaining its position as it becomes nearer 

 to or more distant from the retina. The ciliary processes are receiv- 

 ed into depressions with elevations of the hyaloid membrane between 

 them to allow of its expansion as it advances and occupies a more ex- 

 tended space. In fishes whose crystalline is a sphere, surrounded 

 by the vitreous humor and moved by a simple muscle, such an appa- 

 ratus is not wanted and it does not seem to exist. 



When the muscular fibres at the roots of the ciliary processes con- 

 tract, the cihary veins will be compressed, and the processes will be- 

 come erect and expanded ; their sides will recede from each other, 

 the anterior wall of Petit's canal will be lifted forwards and the crys- 

 talline will advance. When the muscular action ceases, the processes 

 will become less turgid, their sides will approach each other, the an- 

 terior wall of the canal of Petit will be pushed back and the crystal- 

 line will resume its situation. 



Different directions may be given to the rays of light proceeding 

 from objects by a change of the position of the crystalline produced 



♦ See Uie elegant plates of Mr. Bauer, Phil. Trans. Lond. 1S2'J. 



