A Course of Twelve Lectures ON THE STRUCTURE AND DEVELOPMENT OF THE VERTEBRATE SKELETON. Recently delivered at the Royal College of Surgeons of England, BY PROFESSOR HUXLEY, F.R.S

1863 The Lancet  
o the skull of osseous fishes, and I pointed out to you in the firs place that the skulls of these fishes are constructed upon wha we may call two principal plans. One plan of constructioi we observed in the pike, where the anterior part of the cranium was considerably narrowed, and where there was at inter-orbital septum ; the other kind of structure being thai which we observed in the carp, where the cavity of the craniun is continued forward, without any such plate-like inter-orbita septum,
more » ... o the anterior extremity of the skull. It will be un derstood that this difference of arrangement of the parts does not involve any difference in essential structure; and, excepi perhaps that the sizes of the eyes may have a good deal to d( with the matter, it is difficult to see why we should have thE one structure rather than the other. Then I went on to men. tion that there were extraordinary differences in the formation of the skull, the skull varying from the excessively flattened figure of the echeneis, or of some of the siluroid fishes, to thE singularly lengthened form of the sword-fish and the fistularia ' : and we found that the extreme elongation of the skull of the sword-fish and Rstularia was produced in two different manners: the elongation of the skull in the sword-fish being almost entirely caused by the prodigious, disproportionate length of the pre-maxilla, nasal bones, and so on ; while, on the other hand, in the fistularia it was produced by the lengthening of the bones of the hinder part of the face, the jaws proper remaining exceedingly short. If time served, the modifications of form, and of connexion, and of detail of the architecture of the fish's skull might occupy our attention for a very long period; but I must content myself with going very briefly over some of the more important modifications. One of the chief of these is the difference observable in the fish's skull in the development of those parts which we call the pre-sphenoid and orbito-sphenoid. You recollect that in the pike, in the early state, there is no distinct pre-sphenoid, the pre-sphenoidal part of the cartilage of the skull not only becoming ossified, and very imperfectly ossified, late in life, but there are no distinct orbito-sphenoids at all. In the carp, on the other hand, there are exceedingly large and well-developed orbito-sphenoids, and these have the same relation as they have in man essentially, meeting in the middle line below, and, by that junction, taking the place of a distinct pre-sphenoid. This complete development of the orbito-sphenoids, and their union in the middle line below, is also exceedingly well seen in the siluroid fishes, which are indeed closely connected with the carps; but it is also obvious in a great many others whose skulls are constructed upon pretty much the same plan as the skull of the pike. You find, for example, in the genus trachicthys there is a very well-developed pair of orbitosphenoids, and a corresponding median part in the middle line which answers to the pre-sphenoid; so that the absence of these parts that we noticed in the pike is by no means to be regarded as in any way a typical character of the osseous fish's skull But more remarkable still are the modifications of the jaw in fishes. I do not mean merely the lengthening or shortening of the jaws, but certain modifications of the parts of the jaws. In the pike we saw that the maxilla was a small bone placed at the sides of the oral aperture, entirely devoid of teeth, and in a great many siluroid fishes we should find that this small bone is reduced to nothing more than a bony rod, which serves as a kind of skeleton to one of the feelers, thrown so completely outside and beyond the mouth, and away from its ordinary function. But there are fishes, such as the trout for example, where the maxilla has pretty much the same characters as it has in the higher vertebrata, or in certain murasnoid fishes, where you have the true maxilla beset with teeth in the same way and occupying the same position as it would have in the higher vertebrate animals ; so that again there is nothing essential in that peculiarity of the fish's skull. There are certain fishes, amongst which the most prominent, perhaps, are the balistoid, which Cuvier erected into a separate group, in which the pre-maxilia , becomes quite strongly fixed to the anterior part of the skull, and in these fishes it not unfrequently happens that the hinder part of the suspensorial arch becomes firmly fixed, so that that mobility characteristic of most osseous fishes is quite lost in this; but that involves, of course, no essential change in the arrangement of the parts. More remarkable still are the singular distortions which the heads of fishes undergo-of some fishes at least; and there is a symmetrical distortion and an unsymmetrical distortion to be noted. Of symmetrical distortions one of the most curious is that in the light-horseman fish, so called, I suppose, from having a great crest-like helmet at the top of the head, though that would rather assimilate it to a heavy dragoon than a light horseman in the proper sense of the word. This great crest and the large plate attached to it are a development of supra-occipital and frontal bones, which are produced, as may be seen in a section, to a most enormous mass of bone, which is spongy for the most part, but becomes dense, hard, and ivory-like outside. Of course this involves no essential change-in fact, no essential change at all ; it is simply, as it were, a pulling out of this particular region of the skull. In considering the section of this skull, I am reminded of what I ought to have said in speaking of modifications of the interior of the skull, that a great many of the osseous fishes (this is a point of much importance in our future comparisons) have in the lateral walls of the skull-that part which in the pike is united together with the ex-occipital, which lies in front of the exit of the eight pair, and which shelters the hinder part of the external semicircular canal and the hinder part of the posterior semicircular canal. That which we found represented by a distinct ossification in man, they have in the condition of a distinct bone. It is well worth dwelling upon this point, because I shall have to refer to it particularly in speaking of the reptilian skull, and it will be of great moment for us to trace this bone up in the vertebrate series. If you examine the vertical section of the light-horseman's skull, you will find the ali-sphenoid in its ordinary place, the basi-sphenoid, and the prootic bone. The prootic bone is connected above with the ossified post-frontal, which is, in truth, a dismemberment of the prootic. Then behind there comes the ex-occipital, and between the ex-occipital and the prootic is to be seen, inside the skull, a little square portion of bone which belongs to a bone which projects much further upon the outer part of the skull. Then above, joining with this latter bone and with the ex-occipital, and separated from the prootic by the post-frontal,-a dismemberment of the prootic, -there is placed the epiotic. The opisthotic bone shelters part of the horizontal semicircular canal and part of the posterior vertical semicircular canal. It is rare, however, to find it so exceedingly well developed as in the light-horseman. In the cod it is a very large bone lying upon the outside of the skull-more particularly on the outside, still having the same relation with the organ of hearing, but not sending inwards such a process as in the fish I have been speaking of,-forming a sort of pillar round which the external semicircular canal is placed. In other fishes-in the perch, for example-it exists quite as a little ossification in the outer wall of the organ of hearing, but its relations remain essentially the same. In the perch it is that bone to which Cuvier applied the name of petrosal, and various other names have been given to it. Not only in the pike, but also in the carp and in a great many other fishes, this does not exist as a distinct bone; but it appears at a very early period to be completely coalescent with the ex-occipital. It is a very interesting circumstance to find it in a definite form. There is one other kind of asymmetry in the fish's skull which is very much more remarkable-rather, I should say, one kind of distortion, for this is an asymmetrical distortion, not a symmetrical one. This is a very rare thing in the animal kingdom, and met with in but few cases-viz., in the flat fishes to which I am now referring, turbots, flounders, soles, and so on; and it is also to be found to a certain extent, though developed in a different manner, in the cetacea. You are all aware
doi:10.1016/s0140-6736(02)66151-3 fatcat:s3lghfdgnrfrtnvxgozuro4fh4