TBANSACTIONS OF SECTION D. 759 



special set of vessels to bring to it a stream of pure Wood, veins to remove the 

 impure blood, and a tbird set of vessels, tbe lymphatics, concerned in preserving 

 the retina in a state of healthy vitality. 



The retina forms tbe innermost layer of tbe eyeball, and in shape is a hollow 

 sphere with a bole in front to allow tbe rays of light to impinge against tbe 

 posterior part of the membrane. It is this shape of the retina that has hitherto 

 prevented its demonstration with tbe oxy-bydrogen lantern, owing to tbe difficulty 

 of bringing the parts into one plane. This ditficulty I have, however, overcome by 

 carefully stretching the peripheral parts of tbe retina of the ox and other animals, 

 and preparing as a lantern-slide. 



As the central artery appears at tbe optic disc, it gives to the lower part of tbe 

 retina from two to four branches. Of these branches two pass more or less hori- 

 zontally, one going towards the nasal and tbe other towards the temporal side. 

 I'rom these two vessels branches pass right and left until the stem is ex- 

 hausted. Those passing upwards from these two vessels pour tbeir blood into a 

 region whieb may be spoken of as the intermediate zone, where only capillaries are 

 found. In addition to the inferior braucbes mentioned, tbere are generally two 

 others passing downwards. These go to tbe lowest part of tbe retina. Occasion- 

 ally they pass for a short distance as one trunk, wbicb divides into two branches, 

 to be distributed, one to tbe nasal and the other to the temporal side at tbe most 

 inferior part of tbe retina. The veins accompanying these last two vessels are 

 much larger than the arteries, draining tbe blood from a much more extensive iield 

 than is supplied by tbe arteries. After giving off these four brandies, the main 

 artery courses in an almost straight line upwards and outwards. Sometimes, 

 however, it may be seen to be entwined around the corresponding vein once or 

 twice. The main stem gives off two or three branches to the temporal side, and 

 an equal number to the nasal portion of the retina, and sometimes terminates by 

 dicbotomous division. The first two branches given off (one on the temporal side, 

 the other on tbe nasal) send twigs downwards to the intermediate zone already 

 referred to. In nearly all cases the branches of tbe artery given to tbe inner or 

 nasal side are larger than those to the outer side, since tbe inner part of the retina 

 is larger in consequence of the point at wbicb tbe optic nerve pierces tbe sclerotic 

 in tbe ox. 



When tbe specimens are sbown on tbe screen witb tbe aid of the oxy-bydrogen 

 ligbt, tbere is brought out very prominently the fact that, as regards the distribu- 

 tion of vessels, the retina may be dividad into three parts — a lower portion con- 

 taining branches of the four arteries and veins already described, an upper part 

 containing also large vessels, and an intermediate zone entirely free from large 

 vessels except in so far as it is traversed by the main stems of the artery and vein 

 in tbeir course to the upper part of tbe retina. This intermediate zone appears on 

 tbe screen as a wide belt, not unlike a 'watershed' in appearance, but when 

 examined under the microscope, tbe part occupied by capillaries only is much nar- 

 rower than it appears on tbe screen. This zone would seem to be the bomologue 

 of tbe yellow spot found in the human retina. 



Under the microscope, there is seen, throughout the whole extent of the retina, 

 to tbe very margin of the membrane, a dense capillary plexus. The capillaries 

 immediately associated with tbe arteries differ from those directly connected with 

 the veins in that they form irregularly shaped meshes, with tbe appearance of a 

 confused network, whereas those pouring their blood directly into tbe veins have 

 more regularly rhomboidal meshes, and show a general convergence towards the 

 vein in wbicb they terminate. 



In addition to the specimens described, I made preparations of tbe injected 

 retina for vertical sections, and found that the vessels lay chiefly in tbe nerve-cell 

 layer, tbe inner molecular layer, and tbe inner nuclear layer. Only rarely were 

 capillaries found beyond the inner nuclear layer, and they never extended as far as 

 the outer nuclear layer, so that the outer layers of the retina are entirely free from 

 vessels. 



From the foregoing account it is apparent that tbe retina, far from being a 

 membrane witb a few vessels only, as might be inferred from ophthalmoscopic 

 examination, is permeated throughout its whole extent by a dense capillary plexus. 



