876 



OP SENSATION, AND THE ORGANS OF THE SENSES. 



Fig. 209. 



its posterior side rests, to flatten the globe against this, rather than to increase 

 its projection. There is much more ground for the belief, however, that a 

 change of place is effected in the crystalline lens, by the action of the ciliary 

 muscle, and the erectile tissue of the ciliary processes ; for, although no such 

 change can be demonstrated by observation, yet it can be shown that the con- 

 traction of the ciliary muscle (Fig. 209) would tend to draw the lens forwards ; 

 and the fact that this muscle is peculiarly powerful 

 in the predaceous birds, which are distinguished for 

 their great range of vision, and which have, in their 

 circle of osseous sclerotic plates, an unusually firm 

 point of attachment for it, is a strong argument in fa- 

 vor of this doctrine. 1 Further, the almost entire loss 

 of the power of adapting the eye to distances, which 

 is experienced after the removal of the Crystalline 

 lens in the operation for Cataract, is a marked indica- 

 ^ on tnat some change in the place or figure of this 

 body is the principal means whereby the ordinary 

 adaptation is effected ; and although it has been sug- 

 gested that an alteration in the figure of the lens 



Diagram to show the position might participate in the result, yet no means can be 

 and action of the Ciliary Muscle : pointed out as competent to produce it ; so that, as far 



^ WQ cafl fc pregent j udge a change j n the j 

 ., , . \ i < -, i " 



len . s , 1S the 8 <> le means of adapting the eye to 



iary ligament, and point from distinct vision at varying distances. It is certain 

 which the ciliary muscle radiates- that the condition of repose is that of vision for dis- 

 e. iris. n. Lens, connected with tant bj ec t S) no fatigue being experienced from the 



the ciliary processes by the ante- } d direction of the eye to these; whilst the 



rior wall of the canal of Petit, the " . fe _ . . , J > . . 



situation of which is marked by employment of the visual power upon near objects for 



the *. Magnified 3 diameters. some time is accompanied with a sense of effort, and 



is followed by fatigue. The movement which effects 



the change of place of the crystalline lens is performed in obedience to Volition, 

 and is guided by sensation ; yet we are not conscious of performing it, all that 

 we will being the result ; and thus we have another apposite illustration of the 

 really automatic nature of what are termed voluntary movements generally (757). 

 878. When both eyes are fixed upon an object, their axes converge so as to 

 meet in it ; and the degree of convergence is of course altered by variations in 

 the distance of the object ; since, when the object is very remote, the optic axes 

 are virtually parallel, whilst its approach causes them to incline towards each 

 other, and this the more rapidly as the object is brought nearer, the increase 

 being the greatest when it has arrived within the ordinary distance of distinct 

 vision. Here, again, we have an example of the automatic nature of voluntary 

 actions ; for the convergence of the eyes that may be produced by such a gradual 

 approximation of an object on which the eyes are kept fixed by an effort of the 

 Will, far exceeds that which most individuals can induce by an effort made directly 

 for the purpose ; and if, when an object has thus been gradually approximated 

 to within a few inches of the nose, the effort be intermitted and the optic axes 

 be allowed to regain their parallelism, they can seldom be brought to converge 

 again upon it, without repeating the whole process. It has been thought, from 

 the close accordance between the changes required for the adaptation of the eyes 

 to distinct vision at different distances, and the alterations in the direction of 

 the optic axes which are required to bring the two eyes to bear upon objects at 



' See, on this subject, Messrs. Todd and Bowman's " Physiological Anatomy," p. 412, 

 Am. Ed. ; and Dr. Clay Wallace on "The Adjustment of the Eye to Distances," New York, 

 1851. 



-a Sclerotic^. Cornea c. Cho- 

 roid, separated a little from the 



sclerotic, d. situation of the di- 



