630 THE SENSES 



and in most animals small particles of calcareous matter, otoliths. These 

 otoliths, therefore, lie among the projecting hairs of the sensory cells. This 

 is characteristic of these sensory areas and differentiates them from the 

 arrangement present in the cristae. There would seem to be close agree- 

 ment in function between the maculae and cristae, and we naturally look to 

 the influence of the otoliths on the processes which result in the stimulation 

 of the maculae. Attempts have been made to remove the otoliths, with the 

 result that in such animals there Is apparent inability to maintain a constant 

 position in space. The experiments have been performed which have sug- 

 gested the theory that the otoliths take an active part in stimulating the sen- 

 sory cells, probably by their mere pressure. If the head is inclined in one 

 or the other direction, the pressure of the otoliths will shift on the hair cells, 

 and that is sufficient to stimulate them. If this view is correct, then we may 

 regard these structures as static in function as compared with the semicircular 

 canals, which are dynamic. The anatomical separation of the nerves for the 

 cochlea from the division for the utriculus, sacculus, and semicircular canals 

 itself suggests isolation in function, figures 389 and 435. It is conceivable 

 that loud noises of an explosive nature may cause sufficient vibration of the 

 endolymph to affect the otoliths and thus stimulate the cristae. Yet, if such 

 stimulation takes place it is probably only of secondary importance. 



IV. THE SENSE OF SIGHT. 

 THE EYE. 



The eye, the organ of vision, is the most complex and most highly devel- 

 oped of the organs of special sense. It consists not only of a special sen- 

 sory epithelium, the retina, sensitive to light stimulation, but of a series of 

 special structures which intensify and localize the stimulus. There are also 

 accessory organs for the protection of the eye. 



The Eyelids and Lachrymal Apparatus. The eyeball is kept 

 moist over its free surface and protected from external injury by the eyelids, 

 by the glands that secrete the lachrymal fluid to moisten the surface of the 

 cornea, and by the oil glands that secrete oil on the margins of the lids. 



The conjunctiva , or lining membrane of the lids, which is reflected on to 

 the free surface of the eyeball, protects the eye from injury by its extraor- 

 dinary sensitiveness to irritation by dust or other external substance. Its 

 stimulation produces reflex secretion of the lachrymal fluid that flows over 

 the surface of the eye, and tends to wash away the stimulating substance. 



The Eyeball and its Parts. The detail of the structure of the eye- 

 ball is too complex to be given here except in so far as seems necessary for a 

 clearer presentation of the physiological facts. A gross dissection reveals 



