IV. The brain of ornithorhynchus anatinus

Abstract

In the study of a few specimens of the brains of Monotremes and Marsupials, it was discovered that in spite of the published work upon the brain of Ornithorhynchus by such savants as Koelliker (1901), Ziehen (1908), and Elliot Smith (1898-1900), knowledge of the brain stem of this interesting mammal was incomplete, as judged by present-day standards. It was felt that with the introduction of more recent viewpoints, a new study of this subject might chance to give the old facts new relationships. This monograph, as one of the rapidly increasing number of analyses of the brains of vertebrates, may help to clarify in part the inter-relationships between the brains of lower forms and those of mammals. This research, begun at the suggestion of Prof. Elliot Smith, in London, was continued at the University of Chicago, upon material which he most generously loaned the author, and was brought to a conclusion in Cambridge with the use of three series of brains in the collection of Prof. J. T. Wilson.

The Distribution and Morphological Characteristics of Catecholaminergic Cells in the Brain of Monotremes as Revealed by Tyrosine Hydroxylase Immunohistochemistry

“…A series of TH-positive somata were identified in the region of the monotreme brain defined as the fifth arcuate nucleus by Hines [1929]. This region is co-extensive with the region designated as A5 in studies of the CA neurons of other mammals [Kitahama et al, 1994].…”

“…For ease of description and reading, the classical A1-A17/C1-C3 terminology is used throughout the present text, as this is in common usage [Smeets and Reiner, 1994;Smeets and Gonzá lez, 2000]. However, Smeets and Gonzá lez [2000] have indicated that this terminology might not be the most correct, so where the anatomy of the monotremes [Hines, 1929] lacks ambiguity in comparison to other mammals, anatomical names have been given in the heading of the description of each CA cell group.…”

Cynodont Postcranial Anatomy and the “Prototherian” Level of Mammalian Organization

“…The cytoarchitecture of the monotreme brain stem is basically similar to that in other mammals (Noback, 1964). Of particular interest are the corticospinal tracts (in the platypus, see Hines, 1929) which only mammals possess and which mediate impulses relating to the fine coordination of muscular control. Noback and Shriver (1966), observing the variability in funicular distribution of the corticospinal tract in extant orders of mammals, proposed that their common ancestor may have possessed only a diffuse pre-corticospinal system, and that further evolution resulted in the localization of this system into one or another funiculi.…”

The thalamus of the monotremes: cyto- and myeloarchitecture and chemical neuroanatomy

“…It is very small and only recognizable by following the optic tract to it (figures 1m and 7a; Campbell & Hayhow 1972). Hines (1929) mistook it for the bed nucleus of the stria terminalis which lies immediately lateral to it but separated from it by the vena terminalis (figure 8). In the echidna, the dorsal lateral geniculate nucleus is dorsolaterally located as in many other mammals; it is much larger than in the platypus, and its cells tend to form lobular masses that posteriorly merge with those of the more distinct putative lateral posterior nucleus (figures 4i-j and 5j-l ); it descends over the posterior pole of the echidna thalamus towards the ventral lateral geniculate nucleus, so that the relative positions of the two geniculate nuclei there become the same as in marsupials and non-anthropoid mammals.…”

“…In two of the three living monotremes, the platypus and short-beaked echidna, the cytoarchitecture of the thalamus has been described as being rather monotonous, the nuclear divisions being less evident than in marsupials and eutherian mammals. The monotreme thalamus as a consequence has come to be regarded as less differentiated than that of other mammals (Ziehen 1897;Hines 1929;Abbie 1934;Lende 1964;Campbell & Hayhow 1971Welker & Lende 1980;Ulinski 1984). The thalamus of the third living monotreme species, the rare long-beaked echidna, has not been examined.…”