OCR Text |
Show 424 MR. E. W. L. H O L T - S T U D I E S I N [May 1, The organ of the lower orbit-i. e., that belonging to the ocular side of the body, in this case the right side-may be found immediately under the skin overlying the membranous wall of the orbital cavity just behind the eye. To expose it without rupture requires rather careful handling, as, in the fresh condition, the organ is extremely delicate, and it is bound to the skin anteriorly by connective tissue. It is generally of a roughly trihedral outline, occupying a space bounded dorsally by the backward prolongation of tbe interorbital ridge of the skull, anteriorly by the eye, and postero-ventrally by the anterior face of the superficial jaw-muscles of the same side. To expose it fully it is necessary to remove a part of these muscles. The organ will then present much the appearance shown in figure 5 (Plate X X X . ) , according to the state of expansion. Various branches of the V-cranial nerve, which, for the sake of clearness, are omitted from the figure, will be seen in its neighbourhood, mostly passing internal to it. The distribution of these branches varies somewhat in the different species, but usually two small branches pass to the external face of the organ, which receives its nervous supply from one of them. In the Sole the course of the nerves not infrequently affects the shape of the sac, which may assume a bilobate or even a trilobate appearance therefrom. The sac is almost translucent in very fresh examples, save for certain milky-white streaks observable about the periphery. It is smooth externally, aud firmly bound on its inner face to the wall of the orbital cavity. If the internal structure is exposed, by a partial removal of the outer wall, as shown in Plate X X X . figure 6, it will be seen that a number of white muscular bands are attached to the inner surface of the walls, which are otherwise extremely thin and delicate, while other bands traverse the lumen, either independently, or so associated together as to form more or less definite septa. The bands on the walls, while they interlace with each other in all directions, show nevertheless some attempt at a radiate disposition around an orifice which communicates with the orbital cavity itself. There may be more than one such orifice; in fact, in the specimen figured there are two of considerable size (o., o.), while several smaller ones are hidden by the septum (s.). Even in cases where only one distinct orifice occurs, which is perhaps the commonest condition in the Plaice and C o m m o n Sole, I am not certain that minute openings do not also occur in its neighbourhood. The larger opening, or openings, always show a distinctly thickened rim. The specimen figured appeared to be completely subdivided by the septum, but this is not of constant occurrence in the species. Indeed, in speaking of the dextral forms which I have studied, it is only possible to say of the internal structure of the accessory organ of their lower orbit that it is more or less subdivided by muscular septa, and communicates with the orbital cavity by one or more openings, placed close together, of which one is always distinctly larger than the rest. It is advisable, in studying this structure, to partially harden it by an injection of |