676 SPECIAL SENSES 



retina. Above and below, the circle of vision is cut off by the over- 

 hanging arch of the orbit and the malar prominence ; but externally 

 the field is free. With the two eyes, therefore, the lateral field of vision 

 must be equal to at least one hundred and eighty degrees. It is easy 

 to demonstrate, however, with the ophthalmoscope, as well as by taking 

 cognizance of the impressions made by objects far removed from the 

 axis of distinct vision, that images formed upon the lateral- and periph- 

 eral portions of the retina are confused and imperfect. One has a 

 knowledge of the presence and an indefinite idea of the general form 

 of large objects situated outside the area of distinct vision; but when it 

 is desired to note such objects exactly, the eyeball is turned by muscu- 

 lar effort so as to bring them at or near the visual axis. This fact, with 

 what is known of the mechanism of refraction by the cornea and lens, 

 makes it evident that the area of the retina on which images are formed 

 with perfect distinctness is quite restricted. A moment's reflection is 

 sufficient to convince one that in order to see an object distinctly, it is 

 necessary to bring the visual axis to bear upon it directly. 



In examining the fundus of the eye with the ophthalmoscope, the 

 yellow spot, with the fovea centralis, can be seen, free from large 

 bloodvessels and composed chiefly of those elements of the retina which 

 are sensitive to light. If at the same time an image for which the eye 

 is perfectly adjusted is observed, it will be seen that this image is per- 

 fect only at the fovea centralis ; and if the object is removed from the 

 visual axis, there is a confused image upon the retina, removed from the 

 fovea, at the same time that the subject is conscious of indistinct vision. 



At the point of penetration of the optic nerve, the retina is insensi- 

 ble to luminous impressions ; or at least, its sensibility is here so obtuse 

 as to be inadequate for the purposes of vision. This point is called the 

 punctum caecum ; and its want of sensibility was demonstrated many 

 years ago (1668) by Mariotte. 



The existence of this spot which is not in the area of distinct 

 vision gives no inconvenience for the reason that in binocular vision 

 the rays from an object do not fall at the same time on both blind spots. 

 It is not noticed subjectively, even in monocular vision. 



The relative sensibility of different portions of the retina has been 

 found to be, in an inverse ratio, equal to about the square of the dis- 

 tance from the axis of most perfect vision (Volkmann). This observer 

 calculated the distance between the sensitive elements of the retina at 

 which he supposed that two parallel lines would appear as one. In the 

 axis of vision, the distance was 0.00029 i ncn (7-366 /*), and at a devia- 

 tion inward of 8, it was 0.03186 inch (809.244 /i), a diminution in acute- 

 ness, of more than a hundred times. 



