52 DR KNOX on the Comparative Anatomy of the Eye. 



ber of its component tunics, &c. Near its commencement in 

 the bottom of the eye-ball, it adheres very intimately to the in- 

 ner layer of the sclerotic, the one being simply a reflexion from 

 the other. Advancing forwards this union ceases, and they ad- 

 here only at those points where vessels and nerves pass from 

 without to the choroides. In some animals, these pass directly 

 through towards the choroid, but in birds and the deer tribe 

 their course is oblique. We shall return to this whilst descri- 

 bing the peculiarities in the anatomy of the eye of the deer. 

 Still further forward, they are more intimately united by the 

 annulus albus, and here the external layer of the choroid is sup- 

 posed to terminate. I have reason to think, that, in general, it 

 does not terminate, but passes forward with the inner layers of 

 the choroid, to form the ciliary processes and uvea. In fishes, 

 where the annulus albus is quite rudimentary, and does not im- 

 pede the passage of the choroid, or render its termination ob- 

 scure and complex, the external membrane of the choroid is ob- 

 served to pass onward to the edge of the pupil, nor can the most 

 careful dissection, aided by good glasses, demonstrate any addi- 

 tional tunic to exist between it and the transparent covering it 

 receives from the cornea. In some of the mammalia, and in birds, 

 it has appeared to me, that the external layer of the choroid in- 

 includes the annulus albus in part, or perhaps rather that it be- 

 comes looser in texture, and unites with the inner layer of the 

 sclerotic, and so passes forward towards the cornea : in general, 

 however, it seems to unite with the inner layer of the choroid, 

 and to pass forward towards the uvea internal to the annulus 

 albus. When the sclerotic and cornea are carefully removed in 

 the eye of any of the mammalia, the parts seen, and which we 

 may mention in succession, are, the exterior surface of the iris ; the 

 line by which this exterior surface of the iris, near its base, is na- 

 turally connected with the cornea, and which anatomists have 

 called the Circulus niger ; a dark coloured membrane, connecting 



