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Show 28 PROF. T. H. HUXLEY ON CERATODUS FORSTERI. [Jail. 4, physiological significance. In Ceratodus there is the further physiological relation to aerial respiration ; and in all the higher Vertebrata the nasal passages are concerned in sniffing and breathing. With respect to the internal structure of Ceratodus, I shall confine m y remarks, in the present communication, to the brain, the skull, and the pectoral limbs. Ceratodus is, in fact, the most surprisingly suggestive animal I have ever had occasion to study; and the attempt to comprehend the morphological significance of the organs I have mentioned has led m e so far, that I must defer the consideration of other parts of its organization to another occasion. I. The Brain. I had no great hope of finding the brain in a state fit for examination in m y specimen of Ceratodus; and in fact the cerebral substance and that of the nerves are in a very friable condition. But, by great good fortune, the pia mater is so very dense and tough, that it has held the cerebral substance in place; and thus not only the external form, but somewhat of the internal structure of the brain could be satisfactorily determined. Fig. 3. Ceratodus forsteri. Underview of the brain (nat. size). I, olfactory, II, optic, VI, third nerve (?); V, VII, IX, X, roots of the trigeminal, portio dura, auditory, glossopharyngeal, and pneumogastric nerves • v.c, one of the lateral ventricles of the lobus communis, laid open from below.' The brain is represented from above in fig. 1, from the left side in fig. 2, and from below in fig. 3. |