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Show 36 PROF. T.J. PARKER ON CARCHARODON RONDELETII. [Jail. 18, The urinary bladder (specimen C) is small aud opens into the internal compartment of the cloaca by a small papilla (fig. 17, ur.ap.). 7. The Brain. The brain could only be examined in specimen C, in which, as already mentioned, decomposition was far advanced before the fish came into m y possession. As soon as the cranium could be removed, a saturated solution of corrosive sublimate was poured into the brain-cavity through the foramen magnum, so as to harden the brain in situ. Next day the roof of the cranium was sawn off, and the brain sketched from above (Plate VII. fig. 20) ; a plate of mica was then inserted beneath it, the nerves cut, and the organ transferred to strong alcohol with comparatively little shaking. By using these precautions, and thanks in great measure to the thickness of the pia mater, I was able to make a tolerably accurate examination of the brain, although all the softer abdominal viscera were hopelessly decomposed some days before the brain could be got at. The ence-phalon of the foetal specimen E was also examined (Plate VIII. fig. 29). The adult brain is about 13 cm. long from the anterior boundary of the prosencephalon (Plate VII. figs. 20-22, prosen.) to the posterior end of the metencephalon1 or medulla oblongata (meten.); to this must be added about 12 cm., the length of the olfactory lobes (rhinen.), giving a total length of 25 cm. The greatest width, across the cerebrum, is about 3*5 cm. In the foetus (fig. 29) the brain is nearly 5 cm. long by 2 cm. wide. The main difference between the foetal and the adult brain depends upon the elongation, in the latter, of the medulla oblongata and of the olfactory lobes. As will be seen by comparison of figs. 20 and 29, fully one half of the medio-dorsal region of the metencephalon is covered by the cerebellum in the foetus, hardly more than one sixth of it in the adult. Again, in the foetus, the olfactory bulbs are almost sessile upon the prosencephala, their crura being very short; in the adult, on the other hand, the rhinencephalic crura are of great length. In a female specimen of Lamna cornubica, 135 cm. (4\ ft.) long, I find that the brain resembles that of the foetal rather than that of the adult Carcharodon; the olfactory crura are comparatively short, not longer than the prosencephala, and fully one half of the metencephalon is covered by the cerebellum. In the fresh brain the optic lobes were so completely covered by the cerebellum as not to be visible in a view from above, but after hardening in corrosive sublimate the epencephalon had undergone a slight shrinking, allowing the lateral regions of the lobes to be seen. The vertical height of the entire brain is great in proportion to its width ; probably in the adult Carcharodon the decomposed brain had spread out a good deal under the action of gravity. In the metencephalon of Carcharodon the restiform bodies (fig. 20, 1 Vide T. J. Parker, " On the Nomenclature of the Brain and its Cavities." ' Nature,' vol. xxxv. 1886, p. 208. |
