SYSTEM.
Brain.— In the male Ornithorhynchus Meckel found that the brain weighed two drachms, German weight (about four drachms avoirdupoise), and bore a proportion to the weight of the body as 1 to 130. It was in closed by a pretty strong dura matey, of which the fold corresponding with the bony falx ad hered but slightly to that process. The cere brum weighed one drachm and a half, German; nearly the whole of its superficies was smooth ; a few vascular impressions marked the side of the anterior lobe : its shape was triangular, depressed ; the contracted anterior lobes form ing the obtuse apex of the triangle : the pos terior lobes are wide and cover the corpora bigemina. The surface of the cerebrum is smooth and unconvoluted (fig. 181).
In describing the structure of the cerebral hemispheres Meckel observes, with reference to the most characteristic part of this structure in the .Mammalia, " Corpus callosum adest quidem, sed breve, quam haud quatuor linens longitudine requet, memorabilius etiam videtur, in dimidia duo lateralia, lines medians baud confluentia, esse disjunctum. Equidem sal tern in faciebus sere spectintiblis intemis dilacerationis vestigium invenire potui."— L. c. p. 33.
During my investigations of the structure of the brain in the Marsupial animals,' I had in memory the apparent exception to the bird like condition of the corpus callosum, which the Ornithorhynchus, according to the above description, presented, but which each suc cessive example of the brain of the Marsupial quadruped served to establish more firmly as the rule of structure in the higher order of the Implacental sub-class. It was difficult to believe that in the lower or Monotrematous group, the cerebral organ, which indicates so accurately the true affinities and natural posi tion of the Vertebrate animal, and which fol lows so faithfully the degradation of the general organization of each species, should offer so abrupt an ascent to the cerebral condition of the placental Mammalia, as would be indi cated by a corpus callosum of four lines long in a brain of which the hemispheres measure only fourteen lines in length (German scale).
The strong suspicion of an error in the cele biated anatomists description justified a re serve in acknowledging this exception until the opportunity of testing it by a dissection of a brain of a Monotrematous quadruped should have presented itself; and my doubts as to the great development of the corpus callosum of the Ornithorhynchus were further justified by the indication of its nearer ap proach to the Oviparous type afforded by the simple bipartite condition of the tubercules called quadrigemina.' Well preserved sped
mensofOrnithorhynchus 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 fomix 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 striatum 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 thorhynchus deviates most essentially from the Marsupialia, and offers the most direct step in the descent to the Oviparous type.