242 DR R. BROOM ON THE 



corresponds to the inferior septal ridge of marsupials. A very short distance behind 

 the plane of fig. 16 the inner part of the nasal floor cartilage, or Jacobson's cartilage, 

 becomes much thinner, and the upper part curves outwards and downwards, thereby 

 forming a cavity for the reception of Jacobson's duct, which in fig. 17 we find lying in 

 the concavity thus formed, and opening into the floor of the nasal cavity. In this latter 

 figure we find the outer part of Jacobson's cartilage detached as a little flat plate which, 

 though different in shape, is, there is little doubt, the homologue of the outer bar of 

 Jacobson's cartilage in marsupials. The duct of the organ is seen to have a plentiful 

 supply of vascular tissue, especially on its upper side. A little behind this plane the 

 outer bar becomes united with the lower part of the inner plate, and the organ rests in 

 a sort of " V "-shaped trough. For a short distance Jacobson's cartilage still retains its 

 connection with the outer nasal floor cartilage, then becoming detached, the " V " 

 becomes gradually rounded into the normal " C " shape, and the outer nasal floor 

 cartilage becomes more and more reduced. Fig. 18 represents a section just beyond 

 the point where the naso-palatine canal opens into the nasal cavity, and it will be 

 observed that there is no connection whatever between the canal and Jacobson's organ. 

 The two cartilages of Jacobson are separated by the vomer, and rest on the palatine 

 processes of the premaxillary. The organ has the usual mammalian kidney-shape, and 

 in structure does not differ apparently from the marsupial organ. Along the concavity 

 of the organ there is a rudimentary plexus composed of one large and three or four 

 small blood-vessels. 



From the consideration of the above-mentioned details, it will be seen that there is 

 little to distinguish the Edentate organ from that of the Marsupial. The most striking 

 difference is the opening of the organ into the nasal cavity much in advance of the naso- 

 palatine canal, but this is only an extreme exaggeration of the condition met with in the 

 rat-kangaroo, JEpyprymnns. 



RODENTIA. 



In no order of mammals has the organ and its relations been studied so thoroughly 

 as in the Rodentia, and yet in no order is the difficulty of interpreting its affinities so 

 great. Klein has fully described the details of the anatomy in the rabbit and guinea- 

 pig, while Herzfeld has examined the organ in the rat. I have myself examined the 

 organ in the hare, at birth, two stages of foetal rabbit, the mouse, and the Australian 

 water-rat, Hydromys. 



Though the organ is probably better developed throughout this order than in any 

 other of the Eutheria, there are certain peculiarities both in the organ itself and in its 

 relations which distinguish it from that of any other mammal. At present I will leave 

 out of consideration the minute structure of the organ, the well-developed plexus, and 

 the glandular connections, and confine myself to the study of the relations of the organ 

 and its cartilages. 



