THE GLOBE OF THE EYE 807 



cle involved in this process, is the orbicularis palpebranim which de- 

 rives its innervation from the facial nerve. The closure of the upper 

 lid is, of course, greatly facilitated by gravity. The opening of the 

 eyelids is effected by the muse, levator palpebrarum which raises the 

 upper lid, while the lower lid is carried downward by gravity. The 

 latter may be depressed still further by the contraction of the muse, 

 rectus inferior, because the tendon of this muscle and the inferior 

 tarsus are connected with one another by strands of connective tissue. 

 This extra depression, however, is only made necessary when objects 

 in the lower visual fields are to be observed while the head is held erect. 



In many fish, amphibia and reptilia, the eye is completely covered 

 by a transparent skin, while others, such as the sharks, crocodiles 

 and birds, are in possession of a third lid which moves transversely 

 across the cornea from its inner angle. This so-called nictitating 

 membrane is represented in the mammals by the plica semilunaris. 

 In either case, the eyelids serve primarily as a mechanism of protection 

 against high intensities of light and impacts of different kinds. Under 

 ordinary circumstances, their edges are separated by a cleft measuring 

 about 28 mm. in height. The intervening space is known as the 

 rima palpebraris. Inasmuch as the size of the eyeball does not vary 

 very considerably in different individuals, the fact that an eye appears 

 either large or small, is chiefly dependent upon variations in the height 

 of this cleft. 



The Lacrimal Glands and Their Secretion. — The internal surfaces 

 of the eyelids are lined with mucous membrane, which is reflected 

 upon the anterior aspect of the eyeball. This Uning is known as the 

 conjunctiva and the space between its layers as the conjunctival sac. 

 The latter is, of course, chiefly potential, because the lids are firmly 

 appUed to the eyeball and their surfaces are moistened with the se- 

 cretion of the lacrimal gland. This gland presents a compound 

 tubuloracemose character, and resembles the serous salivary glands.-^ 

 The cytoplasm of these cells contains two kinds of elements, namely, 

 small dark granules and large, clear, vacuolar formations which 

 greatly increase in number during their resting period. If, on the 

 other hand, the secretory nerve of this gland is stimulated or if lacri- 

 mation is evoked by means of pilocarpin, these clear bodies disappear, 

 while the dark granules increase in number. The nerve-fibers inner- 

 vating this gland, are derived from two sources, namely, from the 

 lacrimal branch of the ophthalmic (facial) and from the sympathetic.^ 



This gland occupies the upper and outer extent of the orbital 

 cavity, while its lower surface rests upon the convexity of the eyeball. 

 Consequently, its secretion is poured into the outer and upper recess 

 of the conjunctival sac, whence it is spread by capillarity across the 

 cornea, moistening its surface as well as that of the opposing conjunc- 



1 Noll, Archiv fiir mikr. Anatomie, Iviii, 1901, also: Dobrenil, Dissertation, 

 Lyons, 1907. 



2 Dogiel, Archiv fiir mikr. Anatomie, xliv, 1895. 



