578 DEVELOPMENT OF THE EYE. [SECT. 230. 



there are also pretty distinct indications of loops and free termin- 

 ations. Peculiar nervous coils, measuring 002"' to o'028'", were 

 also observed in one case towards the conjunctiva of the eye-lids; 

 into these a single nerve-fibre generally entered, while the issuing 

 fibres numbered from two to four (see my Micr. Anat., ii. 1, p. 31, 

 fig. 13, a. 3). The distribution of the nerves in the lacrymal appa- 

 ratus is entirely unknown. 



According to a very recent statement of H. Mutter, the musculus orbitalis 

 of certain mammalia is composed of unstriated fibres, and it presents the 

 same structure, also, in the orbital cavity of man. 



§ 230. Physiological Remarks. — The eye-ball is not developed, 

 as a whole, from a single point, but is formed by the meeting 

 together of several formations ; one of them proceeding from the 

 central nervous system, a second from the skin, and a third from 

 the parts situated between these two. In the embryo of the chick, 

 the primitive optic vesicles spring from the first cerebral vesicle or 

 anterior brain, and are visible at the commencement of the second 

 day as two protrusions, which are at first sessile, but subsequently 

 provided with a hollow peduncle, the rudiment of the optic nerve. 

 At the commencement of the third day, the formation of the 

 crystalline lens begins from the skin of the face covering this 

 vesicle, the epidermis becoming thickened internally, and intro- 

 verted in such a way, that the anterior wall of the primitive eye- 

 vesicle is likewise pushed inwards and applied to the posterior wall, 

 so that the cavity of the vesicle completely disappears. A secondary 

 eye-vesicle is thus formed, which accurately embraces the "lens after 

 the manner of a mould, the lens having become, in the meanwhile, 

 separated from the remaining epidermis by a constriction, and 

 lying beneath the surface. Subsequently, however, the vitreous 

 body is developed between the lens and the eye-vesicle in a special 

 new cavity. It is not yet ascertained how this structure originates ; 

 but it is most probable, as Scholer states, that it likewise grows 

 inwards from the skin, beginning beneath and behind the lens, 

 and being concerned along with it in the introversion of the pri- 

 mitive eye-vesicle. According to Remak, the retina is formed 

 from the inner thicker wall of the inverted or secondary eye- 

 vesicle; while from the outer thinner wall the choroid coat arises, 

 and subsequently the iris, from its anterior border. The sclerotic 

 and cornea become applied from without, when the eye-ball has 

 reached this stage; and the former is, in part, a production from 

 the skin. 



