LATERAL FACES OF THE HEAD. 51 



The Meibomian glands, embedded in the deep face of the tarsus, secrete a 

 sebaceous matter which jirevents the escape of the tears in a normal state and 

 obliges them to follow the natural passages in reaching the exterior. 



The orbicularis palpebral muscle approximates the two lids and the levator 

 palpebra elevates the superior ; the inferior is depressed by its own weight. 



r. The membrana nictitans, or third ei/elid (haw), is a nucleus of very 

 mobile cartilage surrounded by adipose and connective tissue and the conjunc- 

 tiva, situated at the internal canthus of the eye. It becomes continuous with 

 the adipose tissue enveloping the ocular muscles. It has no inherent movements 

 of its own, but mechanically protrudes when the eye is compressed, and liberates 

 the latter from foreign bodies. It glides over the surface of the cornea when the 

 eye is retracted into its orbit, and can be easily rendered visible when the globe 

 is compressed and drawn into the bottom of the ocular cavity. 



3d. Apparatus of Lubrication of the Eye. — This apparatus com- 

 prises the lachrymal gland (15), situated between the orbital process of the 

 frontal bone and the eyeball. It secretes the tears. These are distributed over 

 the surface of the cornea by the hygrophthalmic canals, which open on the inner 

 surface of the superior eyelid towards its temporal angle, and thence they gain 

 the nasal commissure, where they open through the puncta lachrymaUa into the 

 lachrymal ducts. A small tubercle, the caruncula lachrymalk, a dependency 

 of the conjunctiva occupying the inner angle of the eye, directs them towards 

 these two conduits, whence they pass successively through the lachrymal sac and 

 the lachrymal canal to make their exit through the orifice on the floor of the 

 nostril, where they are liberated on the exterior. 



4th. Apparatus of Locomotion. — The movements of the eye are ^ro^ec- 

 tive and functional. The former have for their object the mechanical protrusion 

 of the nictitans membrane ujion the ocular surface by retracting the eye into 

 the bottom of its orbit. It is accomplished by the posterior rectus muscle. The 

 latter elevate, depress, deviate laterally, rotate the eye, and control the size of the 

 pupillary opening of the iris to regulate the quantity of light which enters the 

 eye. Elevation, depression, and lateral inclination of the eye-globe are executed 

 by the contraction of the superior, inferior, external, and internal recti muscles ; 

 rotation, by the superior and inferior obliques, muscles which are physiological 

 antagonists. These muscles are inserted posteriorily into the bottom orbital 

 hiatus and anteriorily on the external surface of the sclerotic. Their anterior 

 insertion can be exposed by making lateral traction on the eye-globe with a 

 tenaculum. 



Method of Procedure in the Examination of the Eye. — 



Long ago, Bourgelat ^ had clearly indicated the precautions to be taken 

 in executing such an examination skilfully. It is, perhaps, needless 

 to recommend that the head should be divested of all harness, more 

 particularly of blinkers, and that the surroundings should be such as 

 not to obstruct the view or communicate abnormal reflections to the 

 eye. The head is turned towards the entrance of the stable, and the 

 eye is carefully inspected in semi-daylight to observe any pathological 



1 C. Bourgelat, Traite de la conformation ext^rieure du cheval, 5e ed., p. 57. 



