^2 
ment of a body without limbs, yet adapted to rapid locomotion and dependent on 
a remarkable development of the muscles of the trunk, was shewn to meet exactly 
the problem of the progression of an animal immersed in water. The control 
and direction of this muscular power were shewn to be dependent on the quick- 
ness and accuracy of vision. The quantity and quality of subaqueous light were 
next commented on, and the variation of light in strata of different depths as 
affected by the different states of the surface waters, e.g., their state of rest and 
motion, their freedom from turbidity, as also by the state of sunlit or clouded 
atmosphere ; in illustration of which the habits of fish and their power of vision 
at different depths were considered. The position of the eyes on the head, the 
direction of the line of vision, and the extent of the sphere of vision in different 
species of fish, came next under review. The question of the immobility of the 
pupil, as involving certain views relating to the perfection of the eye as an optical 
instrument, and to the received theories concerning the accommodation of vision 
to near and distant objects, was fully discussed, and a comparison instituted between 
the immoveable iris of the fish and the moveable iris of man and mammalians. 
The investigations of Brown- Sequard, concerning the contractility of the fishes' 
iris under the direct stimulus of light, were also noticed ; a statement of Gosse 
respecting the iris of the butterfly blenny was also commented on. 
The second part of this essay was devoted to anatomical details of the coats of the 
eye : — 1. the sclerotic and cornea ; 2. the choroid, with its ciliary processes, and 
3. the choroid " gland" receiving especial notice. The several peculiarities of iris ; 
structure were illustrated by drawings and preparations, the latter being more 
fully exhibited in the Zoological Section, on February 8th. Some remarks upon 
the limits of error in, as compared with the certain advantages obtained by, micros- 
copical examination, were made in introducing this portion of the subject. After 
describing the several structures of the iris and ciliary processes, it was shewn 
that, both on anatomical and physiological grounds, the immobility of the iris 
which had been accepted as a fact derived from general observation, was proved 
by the absence of muscle tissue in the iris, and the presence of a considerable 
band of inelastic and non-contractile fibrous tissue, which formed the borders of 
the pupillary opening. The absence of a ciliary muscle and the undeveloped 
condition of the ciliary processes were pointed out, as indicating that no function 
of "accommodation" could be argued on the commonly received theory of a 
change of position by forward traction of the lens, whilst the equally significant 
fact of the fishes' lens being too hard and too inelastic to admit any explanation 
of accommodation by change of the curved surface of the lens, was brought 
forward in proof of the absence of any such accommodation being produced in 
the eye of the fish as is observed in the other vertebrata. With respect to the 
choroid gland. Dr. Fripp, after briefly stating the views of anatomists concerning 
its structure, demonstrated the purely vascular nature of the gland, and expressed 
his disbelief in the explanations hitherto given of the function assigned to it, 
shewing that there existed no relation whatever between the action of the capil- 
laries and vessels of which the gland was composed, and any function of " accom- 
modation." This hypothetical relation was negatived by considering the facts 
already brought forward in disproof of the assumption that any change of place 
