ORGAN OF VISION. 223 



minute foramina, which admit the entrance and exit of the aqueous 

 humour. 



The composition of the vitreous humour, according to Berzelius, 1 is 

 as follows: Water, 98-40; albumen, 0*16 ; chlorides and lactates, 142; 

 soda, with an animal matter, soluble only in water, 0'02. Its absolute 

 weight is fifteen or twenty times greater than that of the aqueous 

 humour. 



3. It was remarked, in the comparison drawn between the eye and 

 a telescope, that a diaphragm exists in the former, called iris, and 

 sometimes uvea. Generally, however, the latter term is appropriated 

 to the posterior lamina of the iris. By some anatomists, the iris is 

 conceived to be a prolongation of the choroid; by others, to consist of 

 a proper membrane, of a muscular character; and, by others, again, to 

 be essentially vascular and nervous; the vessels and nerves being dis- 

 tributed on an erectile tissue. 2 There is, in the views of anatomists 

 and physiologists, much discrepancy regarding the structure and func- 

 tions of this portion of the eye. M. Edwards, 3 of Paris, affirms, that 

 it consists of four laminae, two of which are extensions of laminae, 

 composing the choroid ; a third belongs to the membrane of the aqueous 

 humour, and is reflected over its anterior surface; the fourth is the 

 proper tissue of the iris. M. Magendie 4 asserts, that the most recent 

 anatomical investigations prove the iris to be muscular, and composed 

 of two sets of fibres; the outermost radiating, whose office is to dilate 

 the pupil ; the innermost circular and concentric, for the purpose of 



Fig. 100. Fig. 101. 



Internal View of the Iris. External View of the Iris. 



contracting it. The arrangement of these fibres is represented in Fig. 

 100, which is an internal view of the human iris magnified three diame- 

 ters; and Fig. 101, an external view, exhibiting the surface to consist 

 essentially of a plexus of bloodvessels. Both are taken from the mi- 

 croscopic investigations of Mr. Bauer, and Sir Everard Home. 5 These 

 vessels and nerves are ramifications of the ciliary, the nerves arising 

 from the ophthalmic ganglion and nasal branch of the fifth pair. 



1 Medico-Chirurgical Transactions, iii. 253. 



2 Lepelletier, Physiologie Medicale et Philosophique, torn. iii. p. 158, Paris, 1832. 



3 Bullet, de la Societe Philom., etc., 1814, p. 81. 4 Op. citat., i. 61. 



6 Lectures on Comparative Anatomy, Lond., 1814-1828; and Mr. Bauer, Philosophical 

 Transactions for 1822, p. 78 



