MULLEKIAN FIBRES. BLOOD-VESSELS OF EETINA. 729 



the Mtillerian fibres is still incomplete. Indeed, the minuteness and delicacy 

 of their structure renders their investigation one of the most difficult sub- 

 jects of anatomical inquiry. Heinrich Miiller, to whom science chiefly owes 

 the advance which has of late years been made towards the elucidation of 

 the minute structure of the retina, described radiating fibres, extending verti- 

 cally from the rods and cones to the membrana limitans, interrupted in their 

 course by the corpuscles of the outer and inner nuclear layers, and connected 

 with the nerve-cells. He subsequently recognised the vertical fibres in the 

 internal layers as connective tissue, a view now universally adopted. It 

 appears to be clearly established, that from the pyriform corpuscles at the base 

 of each cone a thread passes inwards to a corpuscle of the internal nuclear 

 layer. It is also stated that more slender threads unite the rods with the 

 deep layers ; and Kolliker represents a thread passing out from a corpuscle 

 of the internal nuclear layer as afterwards dividing into branches, on which 

 are placed corpuscles of the external nuclear layer, and which terminate in 

 rods. This account of the structure seems best to accord with the physio- 

 logical view now very generally held, that the columnar layer is the more 

 immediate seat of the formation of a distinct image in vision, and of the 

 reception of visual impressions from rays of light impinging upon the retina. 

 It is right to state, however, that a different view is taken by Henle, who 

 believes that the rods are free, and that the fibres observed by H. Miiller 

 and Kolliker are artificial products, the result of coagulation by re-agents. 

 Heule regards the retina as composed of an outer part, which he terms the 

 mosaic layer, and which comprises the columnar structures of Jacob's mem- 

 brane, and the external nuclear layer, and is destitute of blood-vessels ; and 

 an inner nervous part comparable to the structure found in the cerebral con- 

 volutions, and consisting of a stratum of nerve fibres and of two strata of 

 nerve cells alternating with granular strata ; the corpuscles of the internal 

 nuclear layer being considered by him as nerve cells of a smaller order than 

 those of the cellular layer. 



5. Vessels of the retina. An artery enters and a vein leaves the retina 

 between the bundles of fibres of the optic nerve. 



The artery (arteria centralis retinae) is an offset of the ophthalmic, and 

 divides into four or five primary branches as soon as it enters the eye-ball. 

 These larger offsets are situated at first on the inner surface of the nerve 

 fibres, but they soon pass between these into the stratum of nerve-cells, 

 where they form a network of very fine capillaries with rather wide meshes, 

 which reaches in front to the ora serrata. 



The vein corresponding to this artery has a similar distribution : it termi- 

 nates in the ophthalmic vein. In animals there is a circular vessel (circulus 

 venosus retina) following the line of the ora serrata. 



Constituents of the retina in the yellow spot. In this part of the retina 

 the several layers above described undergo some modification : the following 

 are the alterations in the strata from without inwards. In the columnar 

 layer, only the cones are present, but they are set close together, and are 

 smaller than elsewhere. The granular layer is absent opposite the fovea 

 ceutralis. The nervous layer is thus modified : the nerve-cells cover the 

 whole spot, like laminated epithelium, and rest internally on the membrana 

 limitans ; but the molecular substance outside them is absent over the 

 fovea centralis ; the nerve-fibres extend only into the circumference of the 

 spot amongst the cells, without forming a layer over it. The fibres of 

 Miiller are found at the circumference but not over the fovea centralis ; they 

 have an oblique, almost horizontal direction, and present a specially nerve- 



