702 HANDBOOK OF PHYSIOLOGY. 



low spot, is the point at which the optic nerve enters the eyeball, and 

 begins to spread out its fibres into the retina. 



The optic nerve passes forward from the ventral surface of the cere- 

 brum toward the orbit inclosed in prolongations of the membranes, the 

 dura mater, arachnoid and pia mater, which cover the brain. The ex- 

 ternal sheath at the entrance of the nerve into the eyeball becomes con- 

 tinuous with the sclerotic, which at this part is perforated by holes to 

 allow of passage of the optic nerve-fibres and the pia mater with the 

 choroid, the perforated part being the lamina cribrosa. The pia mater 

 here becomes incomplete, and the subarachnoid and the superarachnoid 

 spaces become continuous. The pia mater sends in processes into the 

 nerve to support the fibres. The fibres of the nerve themselves are ex- 

 ceedingly fine, and are surrounded by the myelin sheath, but do not 

 possess the ordinary external nerve-sheath. As they pass into the retina 

 they lose their myelin sheaths and proceed as axis-cylinders. ^Neuroglia 

 supports the nerve-fibres in the optic nerve-trunk. In the centre of the 

 nerve is a small artery, the arteria centralis retime. The number of 

 fibres in the optic nerve is said to he upward of 500,000. The axis- 

 cylinders pass on to the retina, turning over the edges of the poms 

 opticus, to be distributed on the inner surface of the retina, as far as the 

 ora serrata, as a layer of optic nerve-fibres, and separated from the 

 hyaloid membrane which contains the vitreous humor to be presently 

 described, by a very thin layer, the membrana limitans interna. 



The retina consists of certain nervous elements arranged in several 

 layers and supported by a very delicate connective tissue. 



The researches of Cajal upon the structure of the retina of verte- 

 brates has shown that this membrane is a much simpler structure than 

 has heretofore been described. Cajal's observations being confirmed by 

 other observers and accepted by neuro-anatomists, it will be safe to give 

 the descriptions here, as representing our present knowledge of the 

 structure of this membrane. 



The retina is a nervous tissue formed essentially of three layers of 

 nerve-cells. From without inward they are: the layer of visual cells, the 

 layer of bipolar cells, and the layer of ganglionic cells. This subdivision 

 is shown in the diagram (fig. 417). These different layers may be sub- 

 divided so as to give the following layers from without inward: 



of vtou.1 ce,,, 



the ,a yer of bipolar cell,. 



5. Internal molecular layer. \ Forming the layer of 



6. Ganglionic layer, with the fibres of the optic nerve, f ganglion cells. 



The layer of visual cells is subdivided, as seen in the figure, into that 

 of the rods and cones externally and that of the external granular inter- 



