322 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



lateral portion of the eye, and this image is the source of light which throws 

 shadows of the retinal vessels on to the rods and cones. 



Circulation of Blood in the Retina. — When the eye is directed toward a 

 surface which is uniformly and brightly illuminated — e. g. the sky or a sheet 

 of white paper on which the sun is shining — the held of vision is soon seen to 

 be filled with small bright bodies moving with considerable rapidity in irregu- 

 lar curved lines, but with a certain uniformity which suggests that their 

 movements are confined to definite channels. They are usually better seen 

 when one or more sheets of cobalt glass are held before the face, so that the 

 eyes are bathed in blue light. That the phenomenon depends upon the circu- 

 lation of the blood globules in the retina is evident from the fact that the 

 moving bodies follow paths which correspond with the form of the retinal 

 capillaries as seen by the methods above described, and also from the corre- 

 spondence between the rate of movement of the intraocular image and the 

 rapidity of the capillary circulation in those organs in which it can be di- 

 rectly measured under the microscope. The exact way in which the moving 

 globules stimulate the retina so as to produce the observed phenomenon must 

 be regarded as an unsettled question. 



We have thus seen that the eye, regarded from the optician's point of view, 

 has not only all the faults inherent in optical instruments generally, but many 

 others which would not be tolerated in an instrument of human construction. 

 Yet with all its imperfections the eye is perhaps the most wonderful instance 

 in nature of the development of a highly specialized organ to fulfil a definite 

 purpose. In the accomplishment of this object the various parts of the eye 

 have been perfected to a degree sufficient to enable it to meet the requirements 

 of the nervous system with which it is connected, and no farther. In the 

 ordinary use of the eye we are unconscious of its various irregularities, shadows, 

 opacities, etc., for these imperfections are all so slight that the resulting inac- 

 curacy of the image does not much exceed the limit which the size of the 

 light-perceiving elements of the retina imposes upon the delicacy of our visual 

 perceptions, and it is only by illuminating the eye in some unusual way that 

 the existence of these imperfections can be detected. In other words, the eye 

 is as good an optical instrument as the nervous system can appreciate and 

 make use of. Moreover, when we reflect upon the difficulty of the problem 

 which nature has solved, of constructing an optical instrument out of living 

 and growing animal tissue, we cannot fail to be struck by the perfection of the 

 dioptric apparatus of the eye as well as by its adaptation to the needs of the 

 organism of which it forms a part. 



Iris. — The importance of the iris as an adjustable diaphragm for cutting 

 offside rays and thus securing good definition in near vision has been described 

 in connection with the act of accommodation. Its other function of protecting 

 the retina from an excess of light is no less important, and we must now con- 

 sider how this pupillary adjustment may be studied and by what mechanism 

 it is effected. The changes in the size of the pupil may be conveniently ob- 

 served in man and animals by holding a millimeter scale in front of the eye 

 and noticing the variations in the diameter of the pupil. It should be borne 



