THE EYE 715 



tioned, the head falls to one or the other side according to the canal, and the 

 animal shows instability of position in that plane. It has been shown that 

 stimulation of a sectioned canal produced reflex movements in that plane. 



Muscular co-ordination is a complex phenomenon and involves the oper- 

 ation of numerous sensory impulses from other organs of the body, especially 

 from the eyes and general skin. Some confusion has arisen from the fact that 

 there are associated with the disturbance in the semicircular canals move- 

 ments of the eyes and head in higher animals, and of the eyes, head, and fins 

 in such animals as fishes the so-called compensatory movements. Without 

 going into details it is sufficient to state that the sense organs of the semi- 

 circular canals probably form only one of the series of sensory structures 

 concerned in the co-ordination of body movements. 



The Utriculus and Sacculus. The utriculus and sacculus each have 

 a sensory area, the maculae, over which there rests in the human ear 

 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. 



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. 



