A TEXTBOOK OF PHYSIOLOGY 



of the eyeball. The rest of the eyeball is an accessory mechanism 

 for focussing the vibrations of the ether upon the retina, in order 

 that light ma}- not only be appreciated, but vision rendered distinct. 

 It is convenient, therefore, first to study the structure of this apparatus,, 

 and also the physical laws which appl} 7 in connection with vision. 



The Anatomy of the Eye. The eye may be compared to a camera, 

 with its framework, its system of lenses, its diaphragm, and its sensitive 

 plate. The framework of the ej'eball consists of an outer tough, 

 opaque, fibrous coat the sclera, or sclerotic. In front, this becomes 

 transparent, and forms one of the system of lenses the cornea. At 

 the back, the sclerotic is pierced by the optic nerve, which then spreads 

 out over the posterior two-thirds of the eyeball to form the sensitive 

 plate the retina. Between the sclerotic and retina lies the black 

 pigmented and vascular coat, the choroid. Within the eye is sus- 

 pended the lens and its supports, dividing the eyeball into two 

 chambers: (1) That between the cornea and the front of the lens 

 the " aqueous chamber " containing a watery fluid (the ' ; aqueous "); 

 (2) that between the posterior surface of the lens and the retina 

 the vitreous chamber containing a glassy, jelly-like mass (the 

 " vitreous humour "). The aqueous chamber is incompletely divided 

 into two by the diaphragm, or iris. To understand the mechanism 

 by which rays of light are rendered - distinct upon the sensitive 

 retina, it is necessary for us to stud} r in detail these accessory parts. 



Accessory Parts and their Functions. 



The Conjunctiva covers the anterior part of the sclera, and is 

 reflected over the inner aspects of the upper and lower eyelids. It 

 consists of several layers of stratified epithelium. The anterior 

 surface of the cornea is covered by a similar epithelium continuous 

 with that of the conjunctiva. The conjunctiva is kept moistened 

 by the secretion of the lachrymal glands the tears. 



The Lachrymal Glands. The lachrymal glands closely resemble 

 in structure a serous gland. They are situated in the upper outer 

 angle of the orbit, and pour out their secretion by several ducts situated 

 on the inner surface of the upper lid. Normally, the amount of 

 secretion is just sufficient to keep the conjunctiva moist, and enable 

 the eyelids to work smoothly. It is drained away by a small orifice 

 placed at the inner angle of the eye, and thence into the lachrymal 

 sac and to the nasal duct which opens into the inferior meatus of the 

 nose. Excessive secretion is induced by foreign bodies acting on the 

 conjunctiva, irritating vapours in the nose, and by painful emotion, 

 leading to the formation of tears. The eyelids, besides moistening 

 the conjunctiva during the process of winking, protect the eyeball, 

 and, by means of the eyelashes, shade the eyes. They consist 

 of folds of skin and areolar tissue, kept in shape by a plate of fibrous 

 tissue the tarsus. In the skin are contained sweat glands, sebaceous 

 (Meibomian) glands, and the eyelashes. Beneath the skin are fibres 

 of the muscle which closes the lids the orbicularis palpebrarum. 



