54: SPECIAL SENSES. 



Pupillary Membrane. At a certain period of foetal life, 

 the pupil is closed by a membrane connected with the lesser 

 circumference of the iris, called the pupillary membrane. 

 This is not distinct during the first months ; but, between the 

 third and the fourth months, it is readily seen, and is most 

 apparent at the sixth month. The membrane is thin, trans- 

 parent, and completely separates the anterior from the pos- 

 terior chamber of the eye. It is provided with vessels de- 

 rived from the arteries of the iris, anastomosing with each 

 other and turning back in the form of loops near the centre. 

 At about the seventh month, it begins to give way at the 

 centre, gradually atrophies, and generally scarcely a trace of 

 it can be seen at birth. 



Retina. The retina is described by anatomists as the 

 third tunic of the eye. It is closely connected with the optic 

 nerve, and the most important structures entering into its 

 composition are probably continuous with prolongations from 

 the nerve-cells. This is the membrane endowed with the 

 special sense of sight, the other structures in the eye being 

 accessory. 



There is probably no special tissue in the organism that has 

 been the subject of anatomical investigations so minute and 

 elaborate as those made of late years upon the retina. This 

 membrane is divided by some observers into no less than eight 

 distinct layers, each of which may be found described in spe- 

 cial treatises, with the greatest minuteness of detail. It is 

 true that a knowledge of the physiological anatomy of the 

 retina is indispensable, in the study of vision ; but, in the com- 

 plex structure of this membrane, there are parts that seem to 

 be the actual recipients of visual impressions, while others 

 are more or less accessory. In our anatomical descriptions, 

 we shall endeavor to avoid the tediousness of unnecessary de- 

 tail, and shall treat elaborately of those anatomical elements 

 only that are directly concerned in the sense of sight. 



If the sclerotic and choroid be removed from the eye un- 



