OCR Text |
Show 426 MR. E. W. L. HOLT-STUDIES IN [May 1, eating with the cavity of the orbit to which it belongs through a foramen in the skull. In all but Rhombus, the organ lies partly in front of the superficial jaw-muscles, immediately under the skin, and partly between those muscles and the skull. In figure 8 the organ of the Halibut is displayed by the removal of the anterior part of the muscles, and by clearing away such part of a great pad of adipose matter as interfered with the view. It is seen that the organ is divisible into an anterior and a posterior limb, the division being due to the effort of the organ to accommodate itself to the available room-viz., in front of and internal to the muscles named. An arrow shows the passage of the anterior limb to a funicular region leading to the foramen through which access is gained to the general orbital cavity. The foramen in question is that between the parasphenoid and the bony bridge formed by the union of the ectethmoid and sphenotic of the blind side; it also gives exit to a palatonasal branch of the V-cranial. The organ of the blind side in its internal structure is similar to that of the ocular side in such forms as Pleuronectes and Solea. It is somewhat noticeable that it is rather larger in the Halibut than in any of the species which possess a definite sac-like organ of the ocular side, but the description given for the Halibut is sufficiently applicable to all the forms enumerated except the Brill. In this species the difference is brought about by the more forward extension of the jaw-muscles, which completely overlie the organ. In consequence it is flattened : it is semicircular in outline, the arc being ventral, and communicates with the orbital cavity by a short narrow neck arising from the centre of its dorsal surface. It is smaller than that of any other species mentioned, but agrees with them in internal structure, and is considerably larger than its fellow of the opposite side. Figure 7 of Plate X X X . shows a dissection which exposes the upper, or left, orbital cavity in the Plaice, a dextral flat-fish, by the removal of a great part of the right and left frontals, including the whole of the bony interorbital septum, and of the right aud part of the left ectethmoid. It is seen that the orbital cavities are n o w separated only by the fibrous band which forms the internal (morphologically ventral) continuation of the interorbital septum. The recti muscles and the optic nerve have been cut through, and the membranous wall of the cavity has been slit along the margins of the eye to allow of the latter being turned forwards. The orbital cavity is thus fully exposed ; it is seen to be roughly ovoidal in shape, with a conspicuous funnel-like depression on the left side near the posterior end. The membranous wall, which lines the whole of the cavity, is continued into this funnel, which is the opening into the accessory organ which we have already studied from the other side. A seeker can readily be passed through the opening, and the organ can easily be injected with fluid through the medium of the orbital cavity. It is worthy of notice that the upper orbital cavity is bounded on most sides by structures which yield little or not at all to pressure. In front and behind |