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Show 1904.] OP THE SPOONBILL STURGEON. 27 contain, the gill-rakers differ very greatly in their chemical composition either from bone or dentine. I found that the gill-rakers, after being decalcified and then boiled for a few minutes in a strong solution of potassium hydroxide, retained their general form, but the only sign of structure visible in them was a coarse fibrous groundwork. After a prolonged treatment with potash this disappeared, and all that remained was a small amount of a gelatinous precipitate. If the shaft of a gill-raker be thinned by rubbing down on the surface of a fine hone, and then mounted in Canada balsam, a good deal of its minute structure can be made out. When viewed under a magnifying-power of 40 diameters, it is seen to consist of a transparent, faintly yellowish ground-substance, scattered through which are great numbers of lacunae with canaliculi radiating from them. Running through the matrix or ground-substance in a longitudinal direction are yellowish-brown canals which contain blood-capillaries (PI. II. fig. 6). In thin sections the ground-substance appears quite colourless, and when seen under a fairly high magnification indications of lamination are visible in it at the periphery, but they die out towards the centre. The lacunae are for the most part slightly larger than those which are seen in a section of a human long-bone, and they bear no definite relation to the blood-channels, so that there are no indications of Haversian systems. The canaliculi are not nearly so numerous as those found in typical bone, but they are more frequently branched, and, moreover, they principally arise from the two opposite ends of a lacuna, and those belonging to one lacuna freely anastomose with those of several of the neighbouring ones (fig. 7). A very marked tendency is exhibited by the lacunae to be disposed with their long axes parallel to the surface of the shaft with their canaliculi running in a similar direction. The channels containing the blood-capillaries, which are most numerous towards the base of the shaft (fig. 6), take the form of longitudinal canals anastomosing with one another by means of short lateral connections which are given off at frequent intervals. Traced higher up the shaft, the number of these canals becomes considerably less, and the anastomoses with neighbouring canals become fewer. As the extremity is reached they become reduced to two or three trunks, which eventually join with one another just under the extreme point (fig. 8). The exact method of the termination of the channels, however, is not easily to be seen in spirit material, owing to the small amount of blood that is present in the capillary vessels which are contained within them. In the basal portion of the gill-raker vascular channels are entirely absent, but running down the centre is a long, narrow, apparently empty cavity. The matrix is homogeneous, and exhibits in places faint traces of lamination. It is well supplied with lacunae, and many of them differ from those found in the matrix of the shaft in having a knotted or less regular outline, and in being more profusely branched. |
