1891.] ANATOMY OF ORNITHORHYNCHUS. 577 



considers the septa already described to be valves, aad 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 my 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 i-eference 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 dis- 

 tance in relation to tlie 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 " jMamnialian 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. 



We 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 niyer. It is well known to exist in the Ophidia and Lacer- 

 tilia, indeed it is in these groups that it is supposed to attain its 

 maximum development. Its condition in the lower Mammals, and 



39* 



