Du KNOX on the Comparative Anatomy of the Eye. 75 



ly to the base of the iris, and are not so intimately connected 

 with the plexus itself, except by communicating branches. 

 Where very distinct, as in birds and in deer, they send large 

 branches round the iris, imbedded in its substance, and from 

 these arise numerous nervous fibrils, supplying every part of the 

 iris. In those mammiferous animals in which the dissection can 

 be clearly made out, either from the magnitude of the nerves, 

 and the structure of the parts to which these are distributed, as 

 in the deer, or from structure only, as in the horse, we find, that 

 the ciliary nerves may be divided into two distinct sets, viz. one 

 passing to the ciliary muscle, and the other to the iris. The 

 former seem the least numerous, and are in some animals very 

 small ; the latter, with a few exceptions, are tolerably abundant 

 in all the mammalia. 



In the horse, the nerves of the iris do not pass through the 

 ciliary muscle, but under it, *. e. between it and the choroid coat, 

 whilst the nerves supplying the ciliary muscle pass in long chords 

 along the inner surface of the ciliary muscle itself, constantly di- 

 minishing in size, by reason of the numerous fibrils sent to the 

 muscle. We thus discover the cause of the error lately commit- 

 ted by those very excellent anatomists, who fancied the ciliary 

 muscle to be a ganglion, or at least a nervous plexus ; for they 

 saw the ciliary nerves suddenly entering the muscle on their way 

 to the iris, and as it were disappearing ; they perceived, more- 

 over, that other branches seemed to arise from the ciliary muscle, 

 and they hastily concluded, that the intervening substance must 

 be a ganglion or nervous plexus at the least ; but I have shewn 

 this to be a great error ; for the smaller branches of those nerves 

 which enter the ciliary muscle are distributed to it, whilst the 

 larger ones pass directly through, to be expanded in numerous 

 fibrils on the true iris. We may now readily explain the mis- 

 take of those who fancied they discovered nerves proceeding to 



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