130 HUMAN PHYSIOLOGY 



Each retina is a cup-shaped structure functionally 

 subdivided like a mosaic into a vast number of minute 

 areas, every one having its own connection with the 

 optic nerve. The fibers of the optic nerves can be fol- 

 lowed, as we have seen, into the part of the brain- 

 stem which is just below the cerebrum. There they 

 come into relation with nerve cells through which re- 

 flexes can be produced and with other cells which trans- 

 mit impulses to the cerebrum. It is not supposed that 

 any intelligent use of the visual power can be based on 

 the employment of the brain-stem by itself. We believe 

 that the cerebrum must be involved. 



The part of the cortex to which the impulses of visual 

 origin are first sent is as far from the eyes as possible, 

 the rear or occipital region of the cerebrum. The area 

 most certainly used is a portion of the surface where 

 the right and left hemispheres are in contact, the sides 

 of the deep cleft which is between them. Injuries here 

 have caused blindness in many subjects. We know 

 that the central part of the retina has superior usefulness 

 in seeing and it is probable that its connections are cor- 

 respondingly extensive. When sight is recovered after 

 its temporary loss through pressure on the visual cortex, 

 the patient may at first see only in a very small central 

 field and this may gradually widen as the improvement 

 continues. 



After learning of the crossed relation between the 

 cerebrum and the muscles one may expect to hear that 

 the right eye is connected with the left side of the brain. 

 The actual arrangement is less simple. The right half 

 of the right retina, approximately, and the right half 

 of the left retina also, have relations with the right side 

 of the brain. The images on the right halves of the 

 retinas are those of objects toward the left of the ob- 

 server. It follows that destruction of the visual cortex 

 on the right side of the cerebrum will result in loss of 

 vision in the left half of the field. Strictly speaking, 

 it is said that the loss in such cases is a little less than 



