A Course of Twelve Lectures ON THE STRUCTURE AND DEVELOPMENT OF THE VERTEBRATE SKELETON
LECTURE VIL-(Concluded.) I WILL now leave the sharks and rays, and pass on to speak of the sturgeon and spatularia, which also possess, as I have already said, a cartilaginous cranium ; but instead of that cartilaginous cranium being encrusted with minute polygonal plates of osseous matter joined one to the other, they have definite ossifications upon their surface-ossifications which resemble very much in their arrangement the bones which I have described to you in ordinary osseous fishes. If
... ou take a sturgeon's head and soak it in a little boiling water, you will soon be able to separate not only all the upper bones, but also the basal bones-the great bone which runs along the basal portion of the skull and the vomer-just as the bones of the pike were stripped off; and so much further does the process go than in the pike, that you have left no bony matter at all in direct connexion with the skull, but the total result would be a cartilaginous mass, which lies in the interior of the skull Fig. 49 is the Cartilaginous skull of sturgeon: side view. a, Occipito-' cervical mass. b, Vertebrae. c, Parietal region. d, Pre-' frontal ditto. d*, Ethmo-vomerine rostrum. e, f, Upper and lower parts of hyo-mandibular. g, Symplectic. 3e, Mandible. i, Ic, Upper dentigerous cartilage. l, Palatine. m, Maxillary. n, Small separate bone. Below n i is the pterygoid. : primordial cartilaginous cranium of the sturgeon represented in a lateral view ; and you perceive it is composed upon essentially the same type as that of the pike. Above and behind e (Fig. 49 ) is the otic enlargement in the sturgeon's cranium, the cavities in which the semicircular canals are placed: there are the same in the pike's skull. There are the post-orbital processed in the pike, and there are the post-orbital processes in the sturgeon (below c) ; there are the pre-orbital or ant-orbital processes in the pike, and there are the same parts in the sturgeon (d); there is the nasal cavity in the pike, and there is the nasal cavity in the sturgeon (in front of d); there is the long rostrum in the pike, and there is the long rostrum in the sturgeon (d*). So that you see there is a complete correspondence between the two. If you make a section of this primordial or cartilaginous cranium, you find that the cranial cavity in it is comparatively small, but all the relations of the parts are such as I have pointed out to you so often in other cases. There is the cavity in which the brain is lodged; there is the space in which the labyrinth and semicircular canal are placed ; there is the sella turcica ; the exit of the optic nerves ; the olfactory nerves ; the eighth behind the otic capsules; and fifth in front. So that all the relations of the nerves remain exactly as they were He. other cases. But there is a remarkable feature about this iNL which we should not find in the pike's primordial cranmKL. In the first place, the notochord, which runs through the vertebral column of the sturgeon, comes up and remains m ? persistent shape in the base of the skull, so that when the gkall is a little macerated in hot water you can pull out with ease the taper anterior extremity of the notochord which lay in the skull-base, and which terminated just immediately behind the sella turcica. That persistence of the notochord is a t?eoMMitable feature of the sturgeon's skull. But a still more singular peculiarity is the fact that the great maps of cartilage represent* not only the cranium, but a good deal of the vertebrale4lzma as well. At least seven of the anterior vertebr2e are fused tegether into one, forming a continuous tubular mass, under which the basilar bone of the skull extends. We shall dad that this fusion of the vertebrse of the trunk of the skull goec on to a remarkable extent in many fishes ; but hardly & more remarkable extent than in the sturgeon. Then at the hinder part of the sturgeon's skull there are two singular ptvcesses raised up-in truth, a sort of transverse processes of the vertebral column. You must remember that the skull proper ceases at a; that all the hinder part belongs to the v<ii3rtebral column; and that the processes which correspond witk the transverse processes run obliquely upwards and abut agsiua the upper shoulder bones. That is a singular peculiarity of the sturgeon; one, so far as I know, only paralleled amongst osxaoas fishes in the siluroids, where the anterior part of the vertebral column is in the same way fixedly united with the skull, a.1l where the transverse processes of this part of the v6rtehral column ascend in the same oblique fashion and abut 8.gMtM the supra-scapular bone. The ossifications upon the surface <tf. the sturgeon's cranium are exceedingly interesting on aceoulltt of the approximation which they present, not only to til/Me bones which we know as membrane bones in the pike's still. but also to certain bones which in the pike and in man are <sactilage bones. Every one of these bones, you must understand, -the whole of the bony substance of the cranium of the stacgeon,-is entirely developed from membrane. There is yet another singular circumstance about the &bgrtllrgeon-a singular difficulty about the interpretation of the pa,rt,a of the sturgeon's skull. We have all seen in the sturgeon tha.t the upper line of the neck is continued along the body by a series of bony shields. Now, there is every reason to believe that these bony shields are ossifications of the uppermost layer of dermis or true skin. If we take one of these bony shields ( Fig. 50, a) , you perceive that it is as like that which is &e supra-occipital (Fig. 50, b) as possible, and that it has the same texture and position as those two which are the parietals ( Fig. SM, Upper skull plates of stargeon. a, 1st nuchal plate. Middle b, Supraoccipital ditto. L(èteral b, b, b, b, Pre-frontal, frontal, parietal, and supra-scapular plates.