bISEASES OF DOMESTIC ANIMALS. Cl 



it by two or three doses of diuretic medicine, or it will geb better when 

 the animal is able to exercise. 



THE EYE. 



The eye is the immediate organ of vision. It is globular in shape, and 

 is composed of a membraneous sack, in which is contained transparent 

 humours of different densities. The external tunic is the sclerotic and 

 cornea ; the sclerotic covers about four-fifths of the eye. The second is 

 formed of the choroid and iris, a continuation of which forms the ciliary 

 processes. The third or innei^is the retina, which is a nervous coat. The 

 sclerotic is formed of white fibrous tissue ; to this are attached the mus- 

 cles that move the eye. The cornea is transparent, and covers one-fifth 

 of the eye. It fits into the sclerotic as a watch glass fits in its place. The 

 choroid coat is made of three layers, external, middle, and internal. The 

 external vena consists principally of minute veins. The middle layer is 

 formed of the ciliary arteries, forming a plexus. The internal is a pig- 

 mentary layer. The iris is a diaphragm or curtain suspended in the 

 aqueous humour. It varies in color in man ; in the horse it is generally 

 of a light brown color. It is composed of two separate sets of fibres, cir- 

 cular and radiating. If the circular contracts, it contracts the pupil. 

 This is involuntary muscular fibre. In foetal life it is covered by a pupil- 

 lary membrane, which becomes absorbed before or soon after birth. The 

 pupilliary opening is in the centre of the eye, and is elliptical or oval in 

 the horse. There are several humours in the eye ; aqueous, vitreous and 

 crystalline lens. The aqueous is in the anterior and posterior chambers 

 of the eye. It is principally water, and is secreted by the membranes 

 which line the chambers. The vitreous humour occupies about four-fifths 

 of the interior of the eye, and is enclosed in the hyaloid membrane. 

 The crystalline lens is situated between the iris and vitreous humour. 

 It is transparent and bi-convex. Its use is to bring rays of light 

 to a focus. The name given to the small dark bodies just above 

 the pupillary openings is corpora nigra. Ihe retina is an expansion 

 of the optic nerve It consists of nine or ten different layers, com- 

 posed of cylinders, cones, etc. The vitreous humour is about the con- 

 sistency of thin jelly. It is covered by the capsule of the lens, and 

 opacity of one or both of these gives rise to cataract. The appendages 

 of the eye are, eyebrows, eyelids, conjunctiva, muscles, membrana nicti-^ 

 tans, and the lachrymal apparatus ; eyebrows are only rudimentary in 

 the horse. The membrana nictitans tends to throw oft' any oftending 

 matter that may get in the eye. This is composed of fibro-cartilage, and 

 is for the protection of the eye, especially so to those animals that can- 

 not protect it by using the fore extremeties. The conjunctiva is the 

 lining or mucous membrane, and it is continuous with the skin, and is 

 reflected over the sclerotic, and firmly adheres to the cornea. The lach- 

 rymal apparatus is formed of glands and lachrymal ducts, sack, and a 

 small opening in the lachrymal sack. This apparatus secretes and gets 

 rid of the tears, and is called a conglomerate gland. These tears are 

 carried down into the lachrymal sack, pass into the lachrymal duct and 

 down into the nasal opening. In the superior and larger of the eyelids 

 there are glands or ciliary follicles. They secrete a fluid which prevents 

 the adhering of the eyelids during sleep. The muscles of the eye are 

 eight, five of them straight. They are, retractor, abductor, adductor, 

 depressor and levator ; one in the horse and not in man, that which 

 pulls the eye back upon the cushion of fat. There are three oblique 

 muscles — superior, middle and inferior. 



