50 DR KNOX on the Comparative Anatomy of the Eye. 



swell the cornea, by pushing the humours forward, and thus 

 enable the eye to distinguish very near objects. But this theory, 

 though supported by so distinguished an anatomist as BLUMEN- 

 BACH, is by most deemed inadmissible. It appears to me truly 

 wonderful, that such a theory could have stood its ground for 

 any length of time ; for, if it was intended by it to explain the 

 great power of vision in birds, it was unnecessary, since the na- 

 tural form of the eye-ball, and of its contained parts, is in them 

 sufficient to explain the phenomenon ; or, if proposed as a theo- 

 ry, by which to explain the accommodating powers of the eye, it 

 was not less in fault, since, in man, the changes produced in the 

 cornea, at whatever distance an object be placed, are altogether 

 trifling, and nearly inappreciable ; nor have I observed the least 

 alteration in the cornea of birds, whatever might be the distance 

 of the object they viewed. Lastly, Compression of the eye-ball 

 does not cause a protrusion of the cornea, though this experi- 

 ment be made with the eye of the horse, whose sclerotic is com- 

 paratively thin and flexible. During life, the cornea is perpe- 

 tually tense, clear, and, as it were, sparkling, especially in the 

 young and healthy. This is owing to the contraction of the ex- 

 ternal muscles of the eye-ball, an action which persists with life, 

 and is named by physiologists the tonic power of the muscles, in 

 contradistinction to the action caused by volition. After death 

 these muscles relax, the eye-ball is left to itself, and the cornea 

 becomes flat and dim. These appearances have generally been 

 attributed to the escape of the aqueous humour through the 

 cornea, now deprived of life ; but this is a gratuitous and unne- 

 cessary supposition. It was long ago remarked by Dr WHYTT *, 

 that, in apoplexies, whilst the patient still li ved, the eyes lost 

 their lustre, and, in some cases, put on the appearance of those 



See his Physiological Essays. 



