138 SPECIAL SENSES. 



tion is also disturbed, we can see that the varied movements 

 of the globes, by the combined action of the recti and oblique 

 muscles, must correspond for each eye, in the movements of 

 torsion upon an antero-posterior axis, as well as in movements 

 of rotation upon the horizontal or the vertical axis. 



We cannot go more elaborately into the various combina- 

 tions of action of the muscles of the eyeball, without giving 

 the subject more prominence than is desirable in a general 

 treatise on physiology. These questions are minutely studied 

 by Helmholtz, in the work to which we have so often re- 

 ferred. 1 



Parts for the Protection of the Eyeball. 



The orbit, formed by the union of certain of the bones of 

 the face, receives the eyeball, the ocular muscles, the muscle 

 of the upper lid, blood-vessels, nerves, part of the lachrymal 

 apparatus, and contains, also, a certain amount of adipose 

 tissue, which latter never disappears, even in extreme maras- 

 mus. The bony walls of this cavity protect the globe and 

 lodge the parts above enumerated. The internal, or nasal 

 wall of the orbit projects considerably beyond the external 

 wall, so that the extent of vision is far greater in the outward 

 than in the inward direction. As the globe is more exposed 

 to accidental injury from an outward direction, the external 

 wall of the orbit is strong, while the bones which form its 

 internal wall are comparatively fragile. The upper semi- 

 circumference of the orbit, the superciliary ridge, is provided 

 with short, stiff hairs, the eyebrows, which serve to shade the 

 eye from excessive light and to protect the eyelids from per- 

 spiration from the forehead. 



The eyelids are folds of very thin integument, lined by a 

 mucous membrane, the conjunctiva. The subcutaneous con- 

 nective tissue is thin and loose, and is entirely free from fat. 

 It presents numerous short papillae and small sudoriparous 

 glands. At the borders of the lids, are short, stiff, curved 

 hairs, arranged in two or more rows, the eyelashes, or cilia. 



1 HELMHOLTZ, Opiique physiologique, Paris, 1867. 



