COORDINATION 401 



between clear, sharp vision and poor, fuzzy light) and cones (sensitive to colors) (Fig. 

 sight. Accommodation is effected by the ac- 16-6 ) . The cones are crowded around a 

 tion of the ciliary muscle, located near the central region, the fovea centralis, where 

 point of attachment of the suspensory liga- visual acuity is most pronounced. Else- 

 ment which supports the lens (Fig. 16-6). where in the retina the cones are mixed 

 When this muscle contracts the tension of with the rods and vision is not so clear. In 

 the ligament is relaxed and the lens be- order to see clearly, it is necessary to look 

 comes more curved, that is, thicker, which directly at the object so that the image falls 

 brings near objects into focus. There is also on the fovea — all other vision is peripheral 

 a change of internal pressures in the eye- and is less clear. This is readily demon- 

 ball that influence the change in lens curva- strated by attempting to determine detail 

 ture. When the muscle relaxes the ligament while looking to one side of an object, 

 tightens and the pressures are shifted so The rods are more sensitive to light than 

 that the lens flattens out, causing distant the cones and detection of weak light 

 objects to come to focus on the retina, sources is best made when looking to one 

 Therefore, the ciliary muscle is active only side of the source. Fliers search for beams 

 when one is looking at close objects ( under at night with their peripheral rather than 

 30 feet) which is the reason why the eyes foveal vision. Careful observation by scien- 

 can be rested by looking out the window at tists has shown that the immediate stimulus 

 a distant object. is probably chemical, much the same as 



The amount of hght entering the eye is with the senses of smell and taste. The rod 



controlled by a sheet of circular muscular cells of the retina contain a purplish red 



tissue, the iris, which contains the pigment pigment, visual purple ( rhodopsin ) , which 



granules responsible for eye color. Both breaks down into a protein and a substance 



radial and circular muscles are present in called retinene when exposed to light. A 



the iris and it is the antagonistic action of further degradation occurs, producing vita- 



these muscles that do the job of enlarging min A, a famfliar accessory food. The chain 



or constricting the opening, the pupfl. They of chemical events, thus initiated, leads to 



are under the control of the autonomic that physico-chemical condition wliich is 



nervous system and therefore beyond vol- the nervous impulse and which is propa- 



untary control. When the eye is exposed to gated along an optic nerve fiber to the 



bright light the circular muscles contract, brain. 



constricting the pupil, whereas in dim light Since vitamin A is produced upon visual 



the radial muscles contract, causing dfla- purple breakdown, it must also be essential 



tion of the pupil. Thus a delicate arrange- in its formation. People who have a defi- 



ment is provided to project just the right ciency of this vitamin suffer from "night 



amount of light on the retina to obtain the blindness," that is, they cannot see in dim 



best possible picture reception of the exter- light. A person who upon entering a mov- 



nal world. ing picture theater in the afternoon cannot 



Exactly how hght rays are transformed see the individual sitting next to him within 



into the nerve impulses that pass over the fifteen minutes had best increase the 



optic nerve to the brain is only poorly un- amount of vitamin A in his diet, 



derstood at the present time. The conver- Visual purple is derived from the rods 



sion from light energy to nerve energy takes only. It has recently been discovered that 



place in the retina, a very delicate and com- the cones in some reptiles and birds each 



plex structure. It is composed of numerous contain one of four different pigments: red, 



cells, some of which are sensitive to light, orange, yellow, and white. The pigment is 



These are the rods (sensitive to white in the form of a globule resting on the tip 



