776 SPECIAL SENSES. 



tance of about ^ of an inch 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 interlacement of fibres, to the zone of Zinn. In the middle of 

 the membrane, its thickness is about -^ of an inch. It becomes thinner near the ante- 

 rior margin, where it measures only about -^ of an inch. Its external surface is in con- 

 tact with the choroid, and its internal, with the hyaloid membrane of the vitreous humor. 



The optic nerve penetrates the retina about % of an inch within and T ^ of an inch be- 

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

 elevation upon the internal surface of the membrane, perforated in its centre for the pas- 

 sage of the central artery of the retina. At from ^ to of an inch external to the point 

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

 of an inch long and ^ of an inch 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 exactly in the axis of distinct vision. The yellow spot exists only in man 

 and the quadrumana. 



The structures in the retina which present the greatest physiological interest are the 

 external layer, formed of rods and cones, the layer of nerve-cells, and the filaments which 

 connect the rods and cones with the cells. These are the only anatomical elements of 

 the retina, as far as we know, that are directly concerned in the reception of optical im- 

 pressions, and they will be described rather minutely, while the intermediate layers will 

 be considered more briefly. 



Most modern anatomists recognize eight distinct layers in the retina, as follows : 



1. An external layer, situated next the choroid, called Jacob's membrane, the bacillar 

 membrane, or the layer of rods and cones. 



2. The external granule-layer. 



3. The inter-granule layer (cone-fibre plexus, of Hulke). 



4. The internal granule-layer. 



5. The granular layer. 



6. The layer of nerve-cells (ganglion-layer). 



7. The expansion of the fibres of the optic nerve. 



8. The limitary membrane. 



The layer of rods and cones is composed of rods, or cylinders, extending through its 

 entire thickness, closely packed, and giving to the external surface a regular, mosaic ap- 

 pearance ; and, between these, are a greater or less number of flask-shaped bodies, the 

 cones. This layer is about ^ of an inch in thickness at the mid,dle of the retina; ^ 

 of an inch, about midway between the centre and the periphery ; and, near the periphery, 

 about -fjr-Q of an inch. At the macula lutea, the rods are wanting, and the layer is com- 

 posed entirely of cones, which are here very much elongated. Over the rest of the mem- 

 brane, the rods predominate, and the cones become less and less numerous toward the 

 periphery. 



The rods are regular cylinders, their length corresponding to the thickness of the 

 layer, terminating above in truncated extremities, and below in points, which are prob- 

 ably continuous with the filaments of connection with the nerve-cells, though they have 

 been actually traced only into the external granule-layer. Their diameter is about T^TJ-^ 

 of an inch. They are clear, of rather a fatty lustre, soft and pliable, but somewhat brittle, 

 and so alterable that they are with difficulty seen in a natural state. They should be 

 examined in perfectly fresh preparations, moistened with liquid from the vitreous humor 

 or with serum. Their intimate structure, as well as that of the cones, has recently 

 been very closely studied, especially by German anatomists. When perfectly fresh, it 

 is difficult to make out any thing but an entirely homogeneous structure ; but, shortly after 

 death, each rod seems to be divided by a delicate line into an outer and an inner segment, 

 the outer being a little the longer. At the upper extremity of the inner segment, is a 

 hemispherical body, with its convexity presenting inward, called the lentiform body (lin- 



