40 PHYSIOLOGY. 



very few hairs in the under eyelid ; secondly, the eyelids, or 

 palpebrse, upper and under : where they join outwardly, it is 

 termed the external canthus, and inwardly towards the nose, 

 the internal canthus : they cover and defend the eyes. The 

 cartilaginous margin or rim of the eyelid, from which the 

 eyelashes proceed, is named tarsus. In the tarsus and inter- 

 nal surface of the eyelid there are small glands, which secrete 

 a fluid, to prevent friction of the eye and its lids, and facili- 

 tate motion. Thirdly, the lachrymal gland, which is placed 

 on the upper part of the eyelid towards the external canthus ; 

 from this gland the tears are secreted, and conveyed to the 

 inner surface of the upper eyelid by several minute ducts, or 

 canals, named lachrymal ducts. There is another small body, 

 having a glandular appearance, in the inner corner of the eye ; 

 on each side of which there are small orifices, which are called 

 puncta lachrymalia : these are the mouths or openings of 

 two small canals, which, joining together, form a membranous 

 tube ; and this, passing through a small opening in the bone, 

 extends to the lower part of the nostril, where its termination 

 may be distinctly seen in the horse. As the lachrymal gland 

 is constantly forming tears, it must be obvious that some 

 contrivance is necessary to convey them off, and prevent them 

 flowing over the cheek : this purpose is answered by the canal 

 just described. 



When any irritating matter is applied to the eye, the tears 

 are formed too abundantly to be carried off in this way ; they 

 then flow over the cheek. In the human eye, the puncta 

 lachrymalia terminate in a small sac, from which the lachry- 

 mal duct proceeds : this is not the case in the horse. In the 

 inner corner of the horse's eye is placed a body commonly 

 termed the haw, no resemblance to which is to be found in 

 the human eye. The horse has the power, by means of the 

 muscles of the eye, to bring the haw completely over its 

 surface ; it serves, therefore, as a second eyelid, and effectu- 

 ally wipes off any dust, hay, or seeds) or other matter which 

 may have fallen upon the eye. The conjunctive membrane, 

 or tunica conjunctiva, lines the inner surface of the eyelids, 



