OCR Text |
Show 1894.] TELEOSTEAN MORPHOLOGY. 429 more potent than the relaxation of the recti and oblique muscles to re-effect its protraction, and here again the elastic nature of the recessus completely supplies the want. The actiou therefore, except in so far as it is, or may be, concerned in the deposition of the orbital fluid, is purely mechanical, and appears to be almost, if not quite, involuntary. I qualify the statement thus because it appears to m e that flat-fish have their eyes very slightly more elevated when expectant of food than at other times. A fish of the genus Pleuronectes usually shows that it anticipates food by bending its body into a crescent shape, the head and tail off the ground, ready for an instant dart; but the greater degree of elevation of the eyes under such circumstances may in reality only signify that a slight contraction of the orbital muscles is characteristic of the resting condition. A n involuntary organ might be expected to show some connection with the sympathetic system, but such a connection I have not found, though I a m far from stating that it does not occur. The only nervous supply with which I a m acquainted is that derived from the V-cranial, but whether from sensory or motor roots, or from both, is a point which I have not yet investigated. W e have seen that the organ is most developed in Pleuronectes, moderately so in Solea, and least developed in Rhombus ; and I can say from observation that the power of elevating the eyes is precisely in the same proportion in those three genera. I have had no opportunity of studying the habits of Hippoglossoides, as the Long Bough Dab is very difficult to obtain in a healthy condition. The habits of the Halibut are also unknown to m e in this respect; but the fact that it shows the maximum development of the left or upper accessory organ and the minimum development of the right or lower organ, amongst the series of forms which I have studied, is very probably due to the greater difference in the level of its eyes, or of its orbital cavities, than in other flat-fishes. The ocular surface of the head is very convex, while the left eye never gets far beyond the ridge, and is consequently at a much lower plane than the right eye. To bring the two eyes to the same level, if that condition is actually attained by the living fish, must certainly need a greater inequality in the elevating apparatus than prevails in any other fiat-fish known to me. The fact that the recessus of the eye belonging morphologically to the blind side is the larger of the two in all the species studied i3 susceptible of a very simple explanation. The eye of the ocular side, the lower eye, has its orbital cavity bounded on the outer side in great part by loose skin and connective tissue, thus allowing whatever elasticity may be possessed by the undifferentiated membranous wall in this region to come into full play when the eye-muscles are contracted. The elasticity of the skin, combining with the pressure of the external element over a surface greater than that of the eye itself, must certainly afford some assistance in elevating the eye as soon as the muscles are relaxed. In the case of the upper orbit, however, no such aid is forth- |