636 COMPARATIVE PHYSIOLOGY. 



ion of the fifth nerve. 5. During sleep. 6. Upon stimulation 

 of the optic or the third nerve, and the corpora quadrigemina 

 or adjacent parts of the brain. 7. Undfer the effects of certain 

 drugs, as physostigmin, morphia, etc. 



Dilatation {Mydriasis). — 1. In darkness. 2. On stimulation 

 of the cervical sympathetic. 3. During asphyxia or dyspnoea. 



4. By painful sensations from irritation of peripheral parts. 



5. From the action of certain drugs, as atropin, etc. The 

 student may impress most of these facts upon his mind by 

 making the necessary observations, which can be readily done. 



Pathological. — As showing the importance of such connec- 

 tions, we may instance the fact that, in certain forms of nervous 

 disease (e. g., locomotor ataxia), the pupil contracts when the 

 eye is accommodated to near objects, but not to light (the 

 Argyll-Eobertson pupil). In other cases, owing to brain-dis- 

 ease, the pupils may be constantly dilated or the reverse ; or 

 one may be dilated and the other contracted. 



Comparative. — The iris varies in color in different groups of 

 animals, and even in individuals of the same group ; while the 

 color in early life is not always the permanent one. 



In shape the pupil is elliptical in solipeds and most rumi- 

 nants. In the pig and dog it is circular, as also in the cat 

 when dilated ; but when greatly contracted in the latter animal, 

 it may become a mere perpendicular slit. 



The iris is covered posteriorly with a layer of pigment (uvea), 

 portions of which may project through the pupil into the an- 

 terior chamber, and constitute the " sootballs " (corpora nigra) 

 well seen in horses, and very suggestive of inflammatory 

 growths, though, of course, perfectly normal. 



OPTICAL IMPERFECTIONS OF THE EYE. 



Anomalies of Refraction. — l. We may speak of an eye in 

 which the refractive power is such that, under the limitations 

 referred to before (page 531), images are focused on the retina, 

 as the emmetropic eye. The latter is illustrated by Fig. 384. 

 In the upper figure, in which the eye is represented as passive 

 (negatively accommodated), parallel rays— i. e., rays from ob- 

 jects distant more than about seventy yards (according to some 

 writers much less) — are focused on the retina ; but those from 

 objects near at hand, the rays from which are divergent, are 

 focused behind the retina. In the lower figure the lens is rep- 



