38 



[111. ARMY HORSE IN ACCIDENT AND DISEASE. 



The communicating portion of this system consists of the cerebro- 

 spinal nerves, which leave the brain and spinal cord in symmetrica] 

 pairs, and are distributed to the voluntary muscles, to the organs of 

 common sensation, and to those of special sense. 



The sympathetic system consists of a double chain of (/<in<//iti 

 (small brains), extending from the head to the coccyx, one chain 

 along each side of the spine, and the two chains connected by nerve 

 cords. The ganglia are also connected to branches of the cerebro- 

 spinal nerves, thus uniting the two systems. The nerves of the sym- 

 pathetic system are distributed to the involuntary muscles, mucous 

 membranes, internal organs, and blood vessels. 



ANATOMY OF THE EYE. 



(Plate VIII.) 



The eye is the organ of sight and is situated in the orbital cavity. 

 It is spherical in shape and is filled with fluid. The front portion, 



PLATE VIII. 



Section of the eye. 



c, Cornea; e, eyelids; /, fluid; ;', iris; 1, crystalline lens; o, optic nerve; p, pupil; r, retina. 



called the cornea, is perfectly clear and admits the light to the back 

 part of the eye, where it strikes the retina, an expansion of the optic 

 nerve; through this nerve impressions are conveyed to the brain. 

 The colored portion or iris, situated behind the cornea, acts as a cur- 

 tain, regulating the amount of light admitted through the central 

 opening, which is called the pupil. The crystalline L us is a small 

 transparent body situated immediately behind the pupil: it is thick 

 in the center and tapers toward the edges. Its function is to draw 

 the rays of light to a focus on the retina. When the lens is diseased 

 and no light can pass through, the animal is said to have a cataract. 

 The eyelids are two movable curtains, superior and inferior, which 

 protect the eye. Themembrana nictitans, or accessory eyelid (haw , 

 is situated near the inner angle between the lids and the eyeball. 



