75-1 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



of the nerve-fibres in the Retina, on account of their minute size, their soft- 

 ness, and the alteration in their characters that takes place during mounting. 

 Although uniformly much smaller than ordinary nerve-fibres, they present 

 considerable diversities in size, the largest of them being only about g^o^h 

 of an inch in diameter, whilst the smallest are no more than from ^ooo^th 

 to snTT/rr.th of an inch. It is considered by Mr. Bowman that, like the fibres 



FIG. 265. 



FIG. 265. Scheiner's Experiment. A, source of light ; E, po- 

 sition of retina in regard to the focus, c, of the rays entering 

 through two apertures in a card, one of which is covered with 

 a colored glass, g, in an emruetropic eye ; n, position of the retina, 

 m n, in a byperruetropic eye; M, position of retina, p q, in a 

 myopic eye. 



FIG. 266. Vertical Section of Retina of the Human Eye ; 1, 

 bacillar layer; 2, outer granular layer ; 3, intermediate fibrous 

 layer; 4, inner granular layer; 5, finely-granular gray layer; 

 6, layer of nerve-ceils; 7, layer of fibres of optic nerve; 8, lim- 

 itary membrane. 



FIG. 266. 



of the Olfactive nerves ( 606), they consist of axis-cylinders without sheaths. 

 They do not form a continuous fibre-layer at the yellow spot. Externally 

 to the stratum of nerve-fibres, which may be called the Optic layer, is a 

 vesicular stratum called the ganglionic layer (Fig. 267, 3), which consists 

 of a finely granular matrix, wherein are imbedded nerve-cells resembling 

 those of the nerve centres, and having, like them, one, two, or a variable 

 number of processes, some of which appear to become continuous with the 

 fibres about to be described ; at the macula lutea all the ganglion-cells are 

 bipolar. It is to these fibrous and vesicular layers of the Retina, which 

 together make up the analogue of the cortical substance of the Cerebrum, 

 that the principal supply of blood is distributed by the minute capillary 

 network which is spread out through their substance. Immediately external 

 to the ganglionic layer is the internal molecular layer (4), or the layer of 

 gray vesicular matter of Mr. Bowman, the peculiar appearance of which is 

 due to the admixture of a very fiue plexus of spongy connective tissue given 

 off from certain radially running supporting fibres, with nerve-fibrils of im- 

 measurable minuteness. This again is succeeded by the layer of inner gran- 

 ules (5), which contains two distinct kinds of nucleated cellular elements, 

 one of which is imbedded in radially arranged nerve-fibres, and the other 

 in similarly disposed fibres of connective tissue ; next comes (6) the external 

 molecular or granulated layer, consisting of a thin layer of fine plexiform 

 tissue, inclosing a few nuclei and smooth cells, with coarser fibres running 

 parallel to the surface of the retina. The external granule-layer (7), which 

 consists of a thin layer of granules, lying in a finely striated matrix of con- 



