234 SENSE OF SIGHT. 



collection of small mucous follicles, which secrete a thick, whitish 

 humour, to fulfil a similar office with the secretion of the Meibomian 

 follicles. It completes the circle formed by those follicles around the 

 eyelids. The rosy or pale colour of the body is supposed to indicate 

 strength or debility. This it does, like other vascular parts of the sys- 

 tem, and in a similar manner. The puncta lachry media are two small 

 orifices, situate near the inner angle of the eye; the one in the upper; 

 the other in the lower eyelid, at the part where the eyelids quit the 

 globe to pass round the caruncula lachrymalis. They are continually 

 open, and directed towards the eye. Each punctum is the commence- 

 ment of a lachrymal duct, which passes towards the nose in the sub- 

 stance of the eyelids, between the orbicularis palpebrarum and tunica 

 conjunctiva. These open, as represented in Fig. 112, into the lachry- 

 mal sac, which is nothing more than the commencement of the nasal 

 duct or ductus ad nasum. The bony canal is formed by the anterior 

 half of the os unguis, and by the superior maxillary bone, and opens 

 into the nose behind the os spongiosum inferius. Through these ex- 

 cretory ducts, all of which are lined by a prolongation of mucous mem- 

 brane, the tears pass into the nasal fossae. 



Dr. Horner, 1 professor of anatomy in the University of Pennsylva- 

 nia, has best described a small muscle, which is evidently a part of the 

 lachrymal apparatus, and to which he gives the name tensor tarsi. It 

 is on the orbital face of the lachrymal sac ; arises from the superior 

 posterior part of the os unguis; and, after having advanced a quarter 

 of an inch, bifurcates; one fork being inserted along each lachrymal 

 duct, and terminating at or near the punctum. It is probable, that 

 the function of this muscle is to keep the punctum properly directed 

 towards the eyeball; or, as Dr. Physick suggested, to keep the lids 

 in contact with the globe. The office, assigned to it by Dr. Horner, of 

 enlarging, by its contraction, the cavity of the lachrymal sac, and thus 

 producing a tendency to a vacuum which vacuum can be more readily 

 filled through the puncta than through the nose, owing to the valves 

 or folds of the internal membrane of the sac is ingenious, but 

 apocryphal. The tensor tarsi muscle is now commonly associated with 

 the name of Horner 2 "muscle of Horner." 



4. PHYSIOLOGY OF VISION. 



The preceding anatomical sketch will enable the reader to compre- 

 hend this important organ in action. In describing the office executed 

 by its various components, we shall follow the order there observed, 

 premising some general considerations on the mechanism of vision ; and 

 afterwards depict the protecting and modifying influences exerted by 

 the various accessory parts: the different phenomena of vision will 

 next be explained; and, lastly, the information conveyed to the mind 

 by this sense. 



In tracing the progress of luminous rays through the purely physical 



1 Lessons in Practical Anatomy, 3d edit., p. 116, Philad., 1836; and General Anatomy and 

 Histology, 6th edit., ii. 425, Philad., 1843. Also. Rosenmxiller's Handbuch der Anatomie, 

 dritte Auflage, Leipz., 1819. 



2 T. W. Jones, art. Lachrymal Organs, Cyclop, of Anat. and Physiol., July, 1840. 



