424 - Multicellular Animals, Especially Man 



merism of the retinene and to recombine the 

 m-form with opsin. 



The short persistence of excitation is im- 

 portant, however. Excitation from a very 

 short flash enduring just 1 microsecond may 

 persist as long as 0.1 second. This makes it 

 possible for the eye to merge successive 

 stimuli, forming a continuous visual image, 

 as in the merging of the successive frames of 

 a moving picture. 



By dark adaptation, the sensitivity of the 

 retina may be increased more than fifty 

 thousand times. During a steady exposure to 

 bright light, a significant part of the visual 

 pigment of the rods in inactive, as a result of 

 the relatively fast processes of breakdown and 

 the relatively slow rate of resynthesis. In the 

 dark, however, resynthesis of rhodopsin pre- 

 dominates, and within somewhat less than 

 an hour maximum sensitivity is achieved. 



Color vision, which is instrumented by the 

 cones, is relatively more complex and less 

 completely understood. Psychological anal- 

 yses indicate that there are three kinds of 

 cones. These are sensitive particularly to 

 blue, green, and red light respectively, with 

 intermediate colors appreciated by virtue of 

 stimulation of two or more kinds simultane- 

 ously. The light-absorbing component like- 

 wise appears to be one or more retinene-pro- 

 tein complexes; but the opsin part of the 

 molecules is not tine same as in rhodopsin. 

 Consequently the photosensitive pigment of 

 the cones is called iodopsin. Whether there 

 are different kinds of iodopsin, correspond- 

 ing to the different kinds of cones, cannot be 

 decided on the basis of present information. 

 The different varieties of color blindness, ap- 

 parently, result from an absence of one or 

 another of the cones, and this, in turn, is de- 

 termined by a deficiency or abnormality in 

 one or more sex-linked genes (p. 498). 



The Blind Spot. Impulses from the rods and 

 cones are conveyed from the retina by two 

 relays of sensory nerve cells (Fig. 23-6). The 

 fibers of the second relay converge to form 

 the optic nerve, which passes out from the 

 eyeball at a point quite near the fovea cen- 



tralis. Owing to a crowding of the nerve 

 fibers at this point, rods and cones are en- 

 tirely absent, and this small area of the retina 

 is called the blind spot, because images fall- 

 ing upon it are not perceived (Fig. 23-5). 



Receptors of Equilibrium. Typically the 

 equilibrium organ of invertebrate animals is 

 a hollow capsule, called the statocyst (Fig. 

 23-7). The statocyst is lined by hair-bearing 

 receptor cells, and contains one or more 

 grains of sand or other solid statoliths. The 

 statoliths are free to fall in any direction, 

 depending upon the position of the animal. 

 Thus as an animal changes its position the 

 statoliths fall upon and stimulate the differ- 

 ent hair cells, initiating the movements by 

 which the animal "rights itself." 



STATOLYTH 



Fig. 23-7. 



Claus.) 



NERVE 



STATOCYST 



Statocyst of a mollusk (Pterotraches). (After 



The lobster and other Crustacea have sand 

 grains as statoliths, and new sand is taken 

 into the statocyst from the environment each 

 time the animal molts. Thus it is possible to 

 introduce iron fillings instead of sand into 

 the lobster's statocyst — in which case the ani- 

 mal will swim upside down, or in any other 

 position, if a magnet is used to counteract 

 the force of gravity. 



The organ of equilibrium in lower verte- 

 brates is a complex structure called the 

 labyrinth. This labyrinth consists of three 

 curved tubes, the semicircular canals, and 

 two small chambers, the saccule and the 

 utricle (Fig. 23-8). The whole system is filled 



