Integrative Systems 207 



pigmented epithelium and its conductor-pole directed toward the 

 developing lens (C, D). So far as the individual rod or cone is con- 

 cerned, there is no "inversion." They retain the primitive polarity of 

 epidermal sense-cells. It is the manifold manipulation of the layer to 

 which they belong that brings them into their final anomalous situ- 

 ation. But, in spite of it, the eye is a highly efficient organ. 



The foregoing description of the eye refers, in some of its particu- 

 lars, to the mammalian eye. The general structure of the eye is the 

 same in all vertebrates, but there are differences in details. 



The median parietal or pineal eyes have been included in the 

 description of the diencephalon (see p. 145). It is to be noted that the 

 retina of a median eye is not "inverted." The retinal cells are so 

 placed that their receptor-poles point toward the source of the stimulus. 



ORGANS OF EQUILIBRATION AND HEARING 



A common ability of animals is to respond to gravitation and main- 

 tain an appropriate orientation of the body in space. In the vertebrates 

 this orienting is accomplished by operation of special sense-organs 

 related to a purely reflex nervous mechanism which automatically 

 effects such muscular adjustments as are necessary to enable the 

 animal to hold any particular posture or maintain the balance of the 

 body during locomotion. The special sense-organs involved in this 

 mechanism are commonly called "static organs." The eyes, however, 

 play an important accessory role in equilibration. 



In all vertebrates the ear is a static or equilibratory organ. It 

 seems possible that in some fishes and some aquatic amphibians equi- 

 libration is its only function, but in all other vertebrates the ear is 

 functionally duplex and structurally differentiated into two regions, 

 one containing the static receptors, and the other containing receptors 

 which are stimulated mechanically by vibrations of the external 

 medium and thus mediate a sense of hearing. 



The essential nervous organ associated with the static and auditory 

 functions is a very thin-walled membranous sac, highly complex in 

 form, filled with a watery fluid and immersed in a similar fluid, all 

 contained within a skeletal capsule which is joined, one upon each 

 side, to the rear of the brain-case. These otic sacs develop by infolding 

 of a pair of thickened regions (placodes) of ectoderm situated close 

 alongside the hindmost of the three primary vesicles of the embryonic 

 brain (Fig. 179). Each infolded placode becomes a simple, spherical, 

 liquid-filled vesicle which (except in sharklike fishes) loses its connec- 

 tion to the superficial ectoderm. In the course of its further develop- 

 ment, the otic vesicle is subjected to an elaborate series of formative 



