226 SENSE OF SIGHT. 



length, which originate all around from the capsule of the vitreous hu- 

 mour ; pass forward over the edge of the lens ; are attached firmly to 

 its capsule, and there terminate. They are unconnected with the ciliary 

 processes, or iris, and he conceives that their contraction will pull the 

 lens towards the retina. The existence of unstriped muscular fibres in 

 them is confirmed by the observations of Wagner, Todd and Bowman 

 and others. 1 



Of late, the ciliary muscle has been described as a grayish semi- 

 transparent ring of non-striated muscular fibres, which covers the out- 

 side of the corpus ciliare ; and, by its contraction, can draw the ciliary 

 processes forwards, and advance the lens. Dr. Clay Wallace, 2 of New 

 York, who was one of the early describers of this muscle, and did the 

 author the favour to demonstrate it to him, is of opinion, that its fibres 

 when they contract compress the ciliary veins, and thus produce tur- 

 gescence of the ciliary processes which occasions the movement of the 

 lens. It appears to be the same muscle as the -tensor muscle of the 

 choroid tensor choroidese of some anatomists. 3 



Such is an anatomical view of the physical part of the eye proper, 

 so far as is necessary for the physiological inquirer. We have yet to 

 consider the most important part of the organ; that which is essen- 

 tially nervous and vital in its action ; and which, as we have seen, goes 

 to constitute one of the membranes of the eyeball the retina. 



The optic nerves second pair of Willis arise from the anterior part 

 of the optic lobes corpora quadrigemina 4 and not, as was at one time 

 universally believed, from the thalami nervorum opticorum. Setting 

 out from this point, they proceed forwards towards the thalami, to which 

 they adhere ; receiving filaments from the corpus geniculatum externum, 

 an eminence a little anterior to, and on the outside of, the corpora ; and 

 from a layer of cineritious substance, situate between the point of junc- 

 tion of the nerve of each side and the eminentise mammillares called 

 tuber cinereum. 5 Proceeding forward towards the eye, the nerves ap- 

 proach, and form a junction at the sella turcica, or on the upper surface 

 of the sphenoid bone. Anterior to this point, they diverge, each pass- 

 ing through the optic foramen to the corresponding eye; piercing the 

 sclerotic and choroid at a point about one-tenth of an inch from the 

 axis of the eye on the side next the nose, where it has a button-like 

 appearance; and expanding to form the whole, or a part of the retina 

 (see page 215). When the optic nerve is regarded from the inside, 

 after removing the retina and choroid, it appears in the form of a cir- 

 cular spot, perforated with small holes, from which medullary matter 

 may be expressed. This is the lamina cribrosa of Albinus. M. Las- 



1 Baly and Kirkes, Recent Advances in the Physiology of Motion, the Senses, Generation 

 and Developement, p. 25, Lond., 1848. 



2 A Treatise on the Eye, p. 53, 3d edit., New York, 1841, and The Accommodation of the 

 Eye to Distances, p. 14, New York, 1850. 



3 Ruete, in Wagner's Handworterbuch der Physiologic, 16te Lieferung, s. 297, Braunsch- 

 weig, 1847. 



4 A Pathological case illustrating this origin, by G. Kennion, M. D., is in Lond. Med. Gaz., 

 Sept., 1838. 



fi Solly, Lond. Med. Gazette, Sept. 24, 1838. 



