ORGANS OF THE BODY. 617 



although followed up in its "broader outlines, has not been minutely investi- 

 gated up to the present. 



REMARKS. (1.) In many of the mammalia we meet with peculiar structures, known 

 as the organs of Jacobson. These consist of tubes, with cartilaginous walls, and are 

 situated in the substance of the palate ; they open into the duct of Stenson. In these 

 is contained a branch of the olfactory nerves. In their texture they resemble much 

 the regio olfactoria (C. Balogh.}. (2.) Bowman's glands may be made out with com- 

 parative ease. (3.) Those delicate fibrillae which spring from the inferior end of the 

 olfactory cells, and those .which are derived from the spreading out of the olfactory 

 nerves, resemble each other in the most complete manner in every respect. But the 

 difficulties connected with investigation in this direction have hitherto rendered it 

 impossible to demonstrate a direct transition of one into the other. On this account 

 we have marked the gap between the two with a line. 



308. 



The organ of sight is made up of the eyeballs and a series of other 

 accessory structures. These consist of membranous parts, as the eyelids, 

 of glands, especially the lachrymal, and of muscles. 



The bulbus oculi (fig. 575) is formed almost entirely of a system of 



Fig. 575. Transverse section of the eye, after Helmholtz. a, the scle- 

 rotic ; 6, cornea ; c, conjunctiva ; d, circulus tenosvs iridis ; e, tunica 

 choroides and membrana pigmenti; f, ciliary muscle; g, processus 

 ciliaris; h, iris; f, optic nerve; i', colliculus options; k, ora serrata 

 retinae; I, lens; m, membrane of De^emft; n. membrana limitans 

 retinae; o, membrana hyaloidea; p, canal of Petit; q, macula lutea. 



capsules. Its posterior and larger segment is constituted by the opaque 

 sclera (a), while the anterior and smaller division is made up by the 

 transparent cornea (b). Internally it is lined by a system of blackened 

 membranes the uvea, consisting of the choroid (e), the ciliary processes 

 (g\ the ciliary muscle (/), and diaphragm, or iris (h). The cavity of this 

 hollow globe is filled with various refracting media. These, among which 

 the cornea may be numbered as the most anterior, are the fluid of the 

 chambers of the eye (anterior to I), the crystalline lens (I), and the vitreous 

 humour (behind Z). The greater part of the cup-shaped expansion of the 

 optic nerve, or retina (i), is covered by the latter. 



