ORGANS FOR THE PROTECTION OF THE EYE. 879 



deviate from its original direction on either side of the heavy line. 4. Three hori- 

 zontal parallel lines, i cm. apart, are drawn, and through the upper and lower 

 ones are drawn short parallel strokes obliquely from above and to the left down- 

 ward and to the right, and through the middle line similar oblique lines from the 

 right above downward and to the left. The three horizontal lines no longer ap- 

 pear parallel. 5. On looking at a bright, vertical line in a dark room, and in- 

 clining the head toward the shoulder, the line appears to be rotated in the oppo- 

 site direction. 



ORGANS FOR THE PROTECTION OF THE EYE. 



The Eyelids. The structure of the eyelids and the arrangement of their 

 component parts are shown in Fig. 309 and the accompanying description. The 

 tarsus is composed not of cartilage, but of a firm connective-tissue plate, in which 

 the Meibomian glands are embedded. They are acinous sebaceous glands that 

 anoint the lid-rriargin. At the basal margin of the tarsus, especially the upper, 

 the acinotubular glands of Krause have their opening close to the transition- 

 fold of the conjunctiva. The conjunctiva covers the anterior surface of the 

 eyeball as far as the corneal margin, the cornea being covered only with the epi- 

 thelium. The conjunctiva on the posterior surface of the lids has a papillary 

 structure in places, the furrows of which have been considered small mucous 

 glands in man and in several mammalia; a sharp distinction between furrows and 

 glands, however, cannot be made. The epithelium is composed of layers of 

 columnar cells, with intervening goblet-cells. Ruminants possess sweat-glands 

 surrounding the cornea; external to the cornea, toward the outer canthus, 

 the pig has simple, glandular, blind sacs. Waldeyer discovered modified sweat- 

 glands at the margin of the tarsus in man. Small lymphatic follicles of the con- 

 junctiva are called trachoma-glands. The lymph-vessels of the conjunctiva are 

 connected with the lymph-spaces of the cornea and sclera. Stohr saw leukocytes 

 wander upon the free surface of the conjunctiva. Krause found end-bulbs in the 

 conjunctiva of the globe, Dogiel at the lid-margin. The secretion of the conjunc- 

 tiva, aside from some mucus, consists of the lacrimal fluid, which can be produced in 

 as large a quantity by the numerous conjunctival vessels as by the lacrimal 

 glands themselves. 



Closure of the eyelids is effected by the orbicularis palpebrarum 

 muscle (facial nerve), the upper lid falling by its own weight. The 

 muscle contracts: (i) voluntarily, (2) involuntarily in individual con- 

 tractions (winking), (3) as a reflex act from irritation of any of the 

 sensory fibers of the trigeminus distributed to the eyeball and the 

 surrounding tissues, likewise from intense stimulation of the retina by 

 light; (4) persistent, involuntary closure occurs during sleep. 



Opening of the lids is brought about by passive dropping of the lower 

 and active elevation of the upper lid by the levator. The unstriated 

 muscular fibers of the lids, which are in a state of tonic contraction, act 

 in the same way by shortening the lid. In looking downward the lower 

 lid is drawn down by bands of connective-tissue fibers running from 

 the fascia of the inferior rectus muscle to the lower tarsus. 



The Lacrimal Apparatus. The straight and freely branching tubules of the 

 lacrimal gland have secreting cells, which are tall when "loaded," and contain 

 a reservoir for the secretion in the fine-meshed protoplasm; and smaller cells 

 entirely filled with secretion in the form of large drops. A dumbbell-shaped 

 pair of rods represents the nucleus in each cell. The secretion is discharged by 

 contraction of the protoplasm. Intercellular secretory passages penetrate be- 

 tween the cells to the level of the nucleus. A terminal intercellular network 

 is formed by exceedingly fine nerve-fibers. The secretory nerves have been 

 described on p. 684. Four or five large, and from eight to ten small, excretory 

 ducts convey the tears into the fornix conjunctivas, just above the outer canthus. 

 The lacrimal canaliculi, with their open extremities, the lacrimal puncta, dip into 

 the lacrimal lake. The ducts are composed of connective tissue and elastic fibers, 

 and are lined by stratified epithelium. Striated muscle-fibers accompany the 

 ducts, and, by their contraction, keep them open. A sphincter surrounding the 



