THE RETINA 831 



vergence of the visual axes of the two eyes on near vision by the recti 

 interni muscles. All these actions are controlled by a common co- 

 ordinating center. 



It is also of interest to note that the activity of this center is under 

 the guidance of the will and is, therefore, controlled by the cerebrum. 

 This must seem peculiar, because the effector, the ciliary muscle, is 

 composed of smooth muscle tissue. It must be admitted, however, 

 that these reactions are usually preceded by visual sensations. In 

 other words, most persons cannot effect these changes in the eyes 

 unless guided by objects in space. This observation might lead us to 

 suppose that the act of accommodation is a simple reflex, just because 

 the motor actions upon which it is based, seem to necessitate certain 

 sensory impressions. In reality this is not true, because we can easily 

 learn to accommodate without first directing our eyes to near and far 

 objects by simply making voHtional efforts in a dark room, or after 

 our eyes have been shaded. Under ordinary conditions, therefore, the 

 process of accommodation belongs to the group of the psychical reflexes 

 or association reflexes. As such, it occupies a position intermediate 

 between the voluntary responses effected by means of striated muscle, 

 and the perfectly involuntary reactions accomplished with the help 

 of smooth muscle. 



CHAPTER LXXI 

 THE RETINA 



The General Structure of the Retina. — The retina is a delicate 

 membrane containing those elements which are absolutely essential 

 for the reception of the rays of light and for the transfer of the impres- 

 sions evoked by them to the center of sight in the occipital cortex of 

 the cerebrum. It occupies the space between the choroid coat and 

 the hyaloid membrane of the vitreous humor, and extends forward to 

 near the posterior margin of the ciliary body. It terminates here in 

 the so-called ora serrata. Its thickness increases gradually from 

 before backward, namely from 0.1 mm. near the ciliary body to 0.4 

 mm. upon the posterior expanse of the eyeball. 



In cross-section it presents eight distinct layers, namely : 



(a) The fiber layer, composed of nerve fibers striving toward their common 

 point of exit, the porus opticus. 



(6) The layer of nerve cells, forming the ganglion nervi optici. 



(c) The inner molecular layer (stratum reticulare int.). 



(d) The inner nuclear layer, formed by bipolar cells (stratum granularum int.). 



(e) The outer molecular layer (stratum reticulare ext.). 

 (/) The outer nuclear layer (stratum granularum ext.). 

 (g) The layer of rods and cones. 



(A) The layer of hexagonal pigment-cells (stratum nigrum). 



