/ESTHESIOLOGY 

 THE SENSE ORGANS AND COMMON INTEGUMENT 



The organs of the senses (Organa sensuum) receive external stimuli and con- 

 duct impulses to the brain which result in sensations of sight, hearing, taste, smell, 

 and touch. They consist essentially of specially differentiated cells (neuro-epithe- 

 lium) and a conduction path which is simple in the more generalized sense organs, 

 elaborate in those which are highly specialized — the eye and the ear. 



THE SENSE ORGANS AND SKIN OF THE HORSE 

 The Eye 



The eye or organ of vision (Organon visus) in the broader sense of the term 

 comprises the eyeball or globe of the eye, the optic nerve, and certain accessory 

 organs associated therewith. The accessory organs (Organa oculi accessoria) are the 

 orbital f ascise and muscles, the 



eyelids and conjunctiva, and tapper eyelid 



the lacrimal apparatus. These 

 structures will be considered in 

 the order in which they may be 

 most conveniently examined, 

 taking the horse as a type. The 

 bony walls of the orbit have 

 been described in connection 

 with the skull; the periorbita, 

 a fibrous membrane which en- 

 closes the eyeball together with 

 its muscles, vessels, and nerves, 

 may be appropriately included 

 in the account of the fasciae. 



Third eyelid 

 Caruncula . \ 



lacrimalis \ '^ 



Medial 



canthus ^. 



/<? 



i"/ 



THE EYELIDS AND CONJUNC- 

 TIVA 



Loiver 

 Fig. 688. — Left Eye of Hohse. 

 9, Zygomatic arch; iO, supraorbital depression; IS, supraorbital proc- 



7, facial crest. (After EUenberger-Baum, Anat. filr Kunstler.) 



The eyelids, upper and 

 lower (Palpebra superior et in- 

 ferior), are movable folds of 

 integument situated in front 



of the eyeball. When closed, they cover the entrance to the orbit and the 

 anterior surface of the eyeball. The upper lid is much more extensive and 

 more movable than the lower one, and its free edge is more coiicaye. The 

 interval between the lids is termed the palpebral fissure (Rima palpebrarum). 

 When the eye is closed, it is an oblique slit about two inches (ca. 5 cm.) 

 in length- when open, it is biconvex in outline. The ends of the fissure are 

 the angles or canthi, and are distinguished as medial or nasal, and lateral or 



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