572 ANIMAL PHYSIOLOGY. 
or adjacent parts of the brain. 7% Under the effects of certain 
drugs, as physostigmin, morphia, etc. 
Dilation (Mydriasis).—1. In darkness. 2. On stimulation 
of the cervical sympathetic. 3. During asphyxia or dyspnea, 
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-Robertson 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. 
OPTICAL IMPERFECTIONS OF THE EYE. 
The defects to be noticed now are common to all human 
eyes, and probably to the eyes of all mammals, though in 
some persons certain of them, as astigmatism, are of so serious 
a character that they require special remedies. 
Spherical Aberration.—The nature of this defect may be best 
learned from an examination of Fig. 414, below. It will be 
seen that rays of light passing through the lens are brought to 
Fig. 414.—Llustrating spherical aberration (after Le Conte). The best image is formed at S, S, 
but is not perfectly sharply defined even here. 
a focus, varying with the point of the lens through which they 
pass, the focusing power of any ordinary convex Jens being 
greater toward the circumference. This defect is believed to 
be corrected in the human eye, at least to some extent, by the 
following: 
1. The iris cuts off the more strongly refracted outer rays. 
2, The corneal curvature is rather ellipsoidal, so that those 
rays farthest from the optical axis are least deviated by it. 
3. The anterior and posterior curvatures of the lens are cor- 
rective of each other. 4. The power of refraction of the lens 
