250 DR KNOX on the Philosophical Anatomy 



cob. In those animals in whose eyes there exists a tapetum, the 

 membrane of Jacob is not absent, as I supposed *, but has the 

 same degree of transparency as the retina. Lastly, In most ani- 

 mals it is of a deeper colour than in man. If to these we add 

 the fact, that it does not possess any bloodvessels, and that the 

 colouring matter is absent in albino animals -j-, most compara- 

 tive anatomists will agree with me in thinking, that in the mem- 

 brane of Jacob, we may perceive a structure, the product of or- 

 ganisation, itself inorganic, and quite analogous to the coloured por- 

 tion of the rete mucosum of the skin. 



III. Of the Annulus albus. 



IN confirmation of a former conjecture as to the importance 

 of this part of the eye-ball, I find that it is sufficiently vascular 

 (perhaps nearly as much so as the iris J), in every portion ex- 



hserentes viderim, in homine, ave, quadrupede. E<e macula In pisce in membrance 

 speciem cmifluunt totamque retinam tegunt."" It is very evident, however, from 

 several passages in the Ekmenta, that, with many others, he confounded, in most 

 instances, the membrane of Jacob with the pigmentum nigrum. 



* Such was also the opinion of HALLER : " In animalibus quadrupedibus ab ea 

 parte abest, in qua tapetum illud lucidum conspicitur. 1 " 



f- " In cuniculo albo pupillis rubris, niger iste mucus desideretur, eaque ipsa est 

 ratio, quare per corneam membranam vasa retinae et choroideae percipias." 



J The hypothesis that the iris is composed entirely of bloodvessels, chiefly ar- 

 teries, is founded on the supposed correctness of some preparations said to be in the 

 possession of PROCHASKA : those I have made, with very delicate injections, nega- 

 tively disprove the hypothesis. 



The mode usually adopted to demonstrate the circular and radiating fibres of the 

 iris merits particular notice. If we hold the iris of man betwixt us and the light, 

 there may be seen, near to its pupillary margin, a portion of its texture of a de- 

 finite breadth, much more opaque than any other portion of the moveable iris. 



