656 SPECIAL SENSES 



way at the centre, gradually atrophies and scarcely a trace of it can 

 be seen at birth. The presence and condition of the pupillary mem- 

 brane often are important as an aid in determining the age of a 

 foetus. 



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 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. 



If the sclerotic and choroid are removed from the eye under water, 

 the retina is seen, in perfectly fresh specimens, in the form of a delicate 

 transparent membrane covering the posterior portion of the vitreous 

 humor. A short time after death it becomes slightly opaline. It ex- 

 tends over the posterior portion of the eyeball to a distance of about T ^ 

 of an inch (1.7 millimeter) behind the ciliary processes. When torn 

 from its anterior attachment, it presents a finely-serrated edge, called 

 the ora serrata. This edge adheres very closely, by mutual interlace- 

 ment of fibres, to the zone of Zinn. In the middle of the membrane, 

 its thickness is about T J0- of an inch (200 /*). It becomes thinner nearer 

 the anterior margin, where it measures only about -g^ of an inch- (80 /-t). 

 Its external surface is in contact with the choroid, and its internal, with 

 the hyaloid membrane of the vitreous humor. 



The optic nerve penetrates the retina about J of an inch (3.2 milli- 

 meters) within, and -^ of an inch (2.1 millimeters) below the antero- 

 posterior axis of the globe, presenting at this point a small rounded 

 elevation on the internal surface of the membrane, perforated in its 

 centre for the passage of 4 the central artery of the retina. At a 

 point y 1 ^ to | of an inch (2.1 to 3.2 millimeters) external to the point of 

 penetration of the nerve, is an elliptic spot, its long diameter being 

 horizontal, about -J of an inch (2.1 millimeters) long and ^ of an inch 

 (0.7 millimeter) broad, called the yellow spot of Sommerring, or the 

 macula lutea. In the centre of this spot is a depression called the 

 fovea centralis. This depression is in the axis of distinct vision. The 

 yellow spot exists only in man and the quadrumana. 



The structures in the retina that present the greatest physiological 

 importance are the layer of pigment-cells, the layer of rods and cones, 

 the layer of nerve-cells, and the filaments that connect the rods and 

 cones with the cells. These are the only anatomical elements of the 

 retina, so far as is known, that are directly concerned in the reception 

 of optical impressions ; and they will be described rather minutely, 

 while the intermediate layers will be considered more briefly. 



