CiL\p. XX] INTERNAL AND EXTERNAL SENSES 421 



The canals are so arranged (Fig. 196) that any movement of 

 the head causes an increase in the pressure of the endol;^Tiiph in one 

 ampulla, and a corresponding diminution in the ampulla of the 

 parallel canal on the opposite side. Thus, a nodding of the head 

 to the right would cause a flow of the endoh-mph from a to 6 in 

 the right anterior vertical canal, but fror& h' to a' in the left poste- 

 rior vertical canal. Hence the pressure upon the hairs is decreased 

 in a, but increased in a'. Such stimulations of the sensory hairs 

 are transmitted by the dendrites of the vestibular nerve, through 

 the cell-bodies of the vestibular ganglion and the axis cylinders 

 of the auditory nerve, to the pons Varolii and thence to the cere- 

 bellum. It is thought that the cerebellum is the centre in the brain 

 which interprets and adjusts the impulses that arise from stimula- 

 tion of the sensory nerves concerned with muscular sense. It is 

 also the centre that interprets and adjusts impulses that arise from 

 stimulation of the vestibular nerve-endings. From this it follows 

 that the cerebellum controls equilibrium. 



SIGHT 



The Adsual apparatus consists of the eyeballs, the optic nerves, 

 and the nerve centres in the brain. In addition to these essential 

 organs, there are accessory organs which are necessary for the 

 protection and functioning of the eyeball. 



Accessory organs of the eye. — Under this heading we class : 

 (1) eyebrows, (2) eyelids, (3) lacrimal apparatus, and (4) muscles 

 of the eyeball. 



Eyebrows. ^ The eyebrows are composed of two arched emi- 

 nences of thickened skin, covered with hairs. They are situated 

 on the upper border of the orbits, and protect the eyes from too 

 vivid light. 



Eyelids. — The eyelids are two folds projecting from above and 

 below in front of the eye. They are covered externally by the 

 skin, and internally by a mucous membrane, the conjunctiva, 

 which is reflected from them over the globe of the eye. They 

 are composed for the most part of connective tissue, which is 

 dense and fibrous under the conjunctiva, where it is known as 

 the tarsal cartilage. 



Arranged in a double or triple row at the margin of the lids 

 are the eyelashes ; those of the upper lid more numerous and 



