864 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



supply of the iris will influence the size of the pupil. Thus, in forced 

 expiration, by which the return of the blood from the head is retarded, 

 the pupil is contracted. So, also, when the intra-ocular pressure is dimin- 

 ished, as by puncture of the anterior chamber, there is less resistance to 

 the flow of blood to the blood-vessels of the iris and the pupil is imme- 

 diately contracted. On the other hand, strong muscular exertion, which 

 leads to the blood flowing freely into the contracting muscles, will produce 

 dilatation of the pupil. 



The size of the pupil may be modified by various drugs. Substances 

 which dilate the pupil are called mydriatics ; those which lead to its con- 

 traction, myotics. Of the former 

 may be mentioned atropine, hom- 

 atropine, duboisine, daturine, and 

 hyoscyamine. They act chiefly 

 by paralysis of the oculo-motor 

 nerve, while also acting slightly 

 upon the dilator fibres, for after 

 complete paralysis of the oculo- 

 motor nerve the moderate dila- 

 tation thereby produced may be 

 intensified by the administration 

 of atropine. Atropine appears to 

 act mainly by a local mechanism, 

 since it produces dilatation of 

 the pupil even after destruction 

 of the ophthalmic ganglion and 

 division of all the nerves of the 

 eye except the optic, and even, 

 according to some authorities, 

 will produce dilatation of the 

 pupil in an excised eye. 



Myotics, of which physostig- 

 mine or eserine is the best known, 

 may produce contraction of the pupil either by stimulation of the 

 oculo-motor nerve or paralysis of the sympathetic. 



2. Visual Sensations. — Our considerations of the action of the organ 

 of vision have thus far dealt simply with physical processes. The rays 

 of light entering the eye have been traced backward through the trans- 

 parent media of the eye until they resulted in the formation of an image. 

 When the rays reach the retina, sensory impulses are excited and are 

 carried through the optic nerve to the sensorium, where they give rise to 

 a sensation. 



The nature of the changes that take place within the retina does not 



Fig. 885.— Diagram op the Formation of 

 the Retina. (Yeo.) 



The fibres of the optic nerve, N, pass along the inner 

 surface of the retina, R, to meet the ganglion cells, whence 

 special communications pass outward to the layer of rods and 

 cones in the pigment layer, p, next the choroid, c. 



