COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 243 



The most striking characteristic of the organ is that it apparently always opens on 

 to the floor of the nasal cavity in its anterior part, and well in advance of the naso- 

 palatine canal, with which it has no connection. This peculiarity has been found in all 

 the rodents yet examined, and from the examination of Dr Beard's sections of the 

 embryo rabbit of 13 mm. described in his recent paper " On Certain Problems in Verte- 

 brate Embryology " I find that the characteristic peculiarity is already distinctly marked 

 even at this early period. 



Lepus. — To illustrate the Rodent type of organ I have taken a foetal rabbit (head 

 length 19 mm.), as in adult animals the great development of the premaxillaries to 

 some extent mar the cartilaginous arrangements. In front the nasal floor cartilage pre- 

 sents no remarkable peculiarity, differing little from the simple marsupial type, except 

 in that the development is confined chiefly to the inner part, which curves up close to 

 the base of the feeble nasal septum. In fig. 36 we see the much compressed nasal 

 cavity passing down and curving into the outer concavity of the nasal floor cartilage, or, 

 as it may be here called, Jacobson's cartilage. Almost immediately beyond this 

 plane the duct of Jacobson is found passing off from the nasal cavity, and occupying 

 the concavity of the cartilage. In fig. 37 we find Jacobson's organ quite sepa- 

 rated from the nasal cavity, and almost surrounded by cartilage. It will be noticed 

 that there is a rudimentary outer bar, on the one side united above, on the other below, 

 resembling more the condition in Dasypus than in Marsupials. On the palate the 

 extreme anterior part of the naso-palatine canal is cut across. Fig. 38 represents a 

 section near the posterior part of the naso-palatine canal. Jacobson's organ and 

 cartilage are found in their normal form, and the cartilages are supported by the pala- 

 tine processes of the premaxillary. Below and on the outer side of the naso-palatine 

 canal is seen a small cartilaginous element, the explanation of the significance of which 

 is the most troublesome problem in the snout of the rodent. The little cartilaginous 

 process passes forwards almost to the front of the canal, supporting it on its outer side. 

 Behind, it supports chiefly the lower wall of the canal, and when the canal opens into 

 the nasal cavity the cartilage forms a true nasal floor cartilage. As it passes still 

 further back it curves inwards and upwards, and forms an inner wall to the lower part 

 of the nasal cavity. Throughout its whole length it is quite unconnected with either 

 Jacobson's cartilage or the alinasal. In the lower mammals the only cartilage with 

 which homology can be claimed is the outer nasal floor cartilage of the Monotremes. 

 In the higher Eutheria, however, we have a somewhat similar cartilaginous development 

 complicated in front by the presence of an anterior process of Jacobson's cartilage, which 

 is absent in the rodents. By a comparison with the simple higher Eutherian type, as 

 found in Miniopterus, it will be seen that the peculiar process of cartilage is a much 

 modified outer nasal floor cartilage. 



From the consideration of the above features it will be noticed that in the rodent we 

 have a number of lower mammalian characters together with what would seem to be a 

 higher Eutherian feature. The well-developed condition of the organ, with its large 



