. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MONOTREMATA. 383 called ' quadrigemina.' Well preserved speci- mens of Ornithorhynchus presented to me by Mr. Thomas Bell, surgeon R. N., in 1838, have ena- bled me to determine this question. There is neither corpus callosum nor septum lucidum in the Ornithorhynchus. The part described by Meckel as the corpus callosum corresponds with the fornix and hippocampal commissure, as it exists in the Marsupialia, excepting that the essential func- tion of the fornix, as a longitudinal commis- sure, uniting the hippocampus major
. The cyclopædia of anatomy and physiology. Anatomy; Physiology; Zoology. MONOTREMATA. 383 called ' quadrigemina.' Well preserved speci- mens of Ornithorhynchus presented to me by Mr. Thomas Bell, surgeon R. N., in 1838, have ena- bled me to determine this question. There is neither corpus callosum nor septum lucidum in the Ornithorhynchus. The part described by Meckel as the corpus callosum corresponds with the fornix and hippocampal commissure, as it exists in the Marsupialia, excepting that the essential func- tion of the fornix, as a longitudinal commis- sure, uniting the hippocampus major with the olfactory lobe of the same hemisphere, is more exclusively maintained in the Ornithorhynchus, in consequence of the smaller size of the trans- verse band of fibres uniting the opposite hip- pocampi, and representing the first rudiment of the corpus callosum, as it appears in the development of the placental embryo. The thin internal and superior parietes of one lateral ventricle are wholly unconnected with those of the opposite ventricle. Meckel makes no mention of the fornix or hippocampus major: the latter forms a large pyramidal prominence at the outer and pos- terior part of the ventricle, and is confluent with the inferior and external parietes of that cavity. The corpus stnatum is long and nar- row : the thalamus opticus small, and is united with its fellow by a soft commissure, which rises to the same level, whereby they appear to form a continuous body. The anterior com- missure is very large, as in the Marsupials. The posterior bigeminal body is much smaller than the anterior, and the trans- verse depression which divides them is very feebly marked: the longitudinal groove is equally feeble on the ' nates,' and is alto- gether absent in the ' testes,' which thus form a single small tubercle. It is in the condition of these parts, recognized, but too briefly no- ticed by Meckel, that the brain of the Orni- tliorhynchm deviates most essentially from the Marsup
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