I—The Brain-Stem and Cerebellum of Echidna aculeata

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Abstract
Descriptions of the brain of Echidna are almost lacking in modern literature. Of the older accounts, those by Ziehen (1897, a and b ; 1901, 1908), Kölliker (1901), and Elliot Smith (1899, 1902) are the most important. More recently, descriptions of isolated structures have appeared in accounts of the phylogeny of these. Thus Van Valkenburg (1911) has described the mesencephalic nucleus and root of the fifth nerve, Kooy (1916) the inferior olive, and Schepman (1918) the eighth nerve. In addition, Fuse (1926, a, b, c, d , and f ) has published a series of papers on various structures in the medulla oblongata including the nucleus arcuatus, the nucleus of the “Zonalbündel,” the “Zonalbündel” itself, the lateral nucleus of the lateral fasciculus of the medulla oblongata, the superior olive, and the tuberculum acusticum. Although no recent paper deals with the fore-brain it is known that the great size of the cerebral hemispheres owes very little to the olfactory system and even less to the visual apparatus. Since any explanation based on the hypertrophy of the trigeminal nerves does not account for the increased size of the cerebrum as contrasted with that of Ornithorhynchus , it is felt that a study of lower centres must precede a detailed investigation of the fore-brain. This study reveals that some of the features of the hemispheres are explicable from the development of the lower centres. An extensive account of the brain of Ornithorhynchus has recently been published by Hines (1929). This has been used as a basis of comparison, and some of the differences in interpretation are the result of the possibility of such comparison. The only other mammals at all closely connected with Echidna are the marsupials. In the works of the American neurologists we have a very complete record of the structure of the brain of Didelphys virginiana and, wherever possible, comparisons have been instituted with their descriptions. No attempt has been made to review the whole of the literature on the various sections described here. Only the most representative papers are quoted and most of these survey the relevant literature very thoroughly. Throughout this work reference was made to the atlases of Winkler and Potter on the brain of the rabbit (1911) and cat (1914) ; in addition, the treatise by Kappers (1920) was freely consulted.