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Show 1891.] ANATOMY OF ORNITHORHYNCHUS. 577 considers the septa already described to be valves, and he concludes that in the absence of any muscles for closing the nostrils such valves must be very useful to an animal with the aquatic habits of the Ornithorhynchus. Various aquatic mammals, such as the Seal, possess a powerful sphincter muscle for closing the nostrils when diving, but I know of no animal with a structure like that in the Ornithorhynchus. The function of these septa is very doubtful. They do not appear in m y sections to be sufficiently large and free to meet one another so as to close the anterior part of the nose: further, it is obvious that were they to do so they would constitute a serious obstacle to inspiration of air through the nose. The Organ of Jacobson. Numerous papers have been published recently on the structure and relations of this organ in various mammals, but scarcely anything appears to be known with reference to its condition in the Ornithorhynchus. Indeed Sir W . Turner (4) and the late Professor W. K. Parker (6) seem to be the only anatomists who have given any account of the organ. Sir W . Turner, when investigating the relations of the naso-palatine foramen to the dumb-bell-shaped bone, ascertained that a bristle passed from the mouth into this foramen entered a cavity which extended backwards for some distance in relation to the nasal septum. Turner considered that this cavity with its walls formed tbe organ of Jacobson. On opening the nasal chamber he found that this organ formed a ridge projecting from the side of the nasal septum halfway across the cavity. Prof. Zuckerkandl (5) also recognized this ridge and figured it (see plate i. fig. 2 of his work), but he did not associate it with Jacobson's organ, and merely stated that it increases the mucous surface and diminishes the anterior part of the nasal cavity. Professor Parker in his "Mammalian Descent" (the Hunterian Lectures for 1884) referred as follows to Jacobson's Organ:-" In my young specimen of Ornithorhynchus (the size of a moderate fist, with the hair appearing) these parts and their capsules are as large as in Serpents and Lizards," p. 52. He gave no further description of the organ in the Ornithorhynchus, but mentioned it as one of the Reptilian characters of the Prototheria. W e know from the researches of Jacobson, Gratiolet, Balogh, Klein, Herzfeld, Schwink, Kolliker, Gegenbaur, and others that Jacobson's organ exists in the great majority of mammals in which it has been looked for. Its structure, however, is comparatively rudimentary, especially in the higher Mammalia. Its existence in man is disputed, and even if present it is very feebly developed. It is absent in Birds, and until recently this was supposed to be the case also in the Crocodilia. Professor Howes (8) has, however, brought before this Society strong evidence of its existence in the Caiman niger. It is well known to exist in the Ophidia and Lacertilia, indeed it is in these groups that it is supposed to attain its maximum development. Its condition in the lower Mammals, and 39* |