COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 235 



say whether certain of the peculiarities presented are due to the specialisation, or are 

 retained primitive characters. 



The condition of the cartilages of the snout may be briefly described as follows. In 

 the hinder part of the nasal region the nasal septum is a short, thick, simple cartilage, 

 which gives off above two well marked alinasals, as in higher mammals. On passing 

 forward, when we reach the plane passing through the anterior part of the prevomer or 

 dumb-bell shaped bone, the nasal septum becomes divided into a very slender upper part 

 supporting the alinasals, and a thick lower part. This lower part is here in close contact 

 with the inner parts of the well developed nasal floor cartilages, while a little in front it 

 becomes united with these, and forms a well developed flat cartilage, stretching right 

 across from the one premaxillary bone to the other. About 3 mm. in front of the plane 

 of the anterior nares the large flat plate of cartilage becomes thickened and arrested in 

 the middle line, but laterally it passes out to the margins of the beak, and sending de- 

 velopments in front and behind forms the support of the beak throughout its whole 

 extent. Though in none of the higher vertebrates is there any cartilaginous develop- 

 ment similar to the large rostral cartilage of Ornithorhynchus, there is reason to believe 

 that it is a very great development of the prenasal element. 



With regard to the nasal floor cartilages, which on passing back are separated off 

 from the median or septal part of the main cartilage, there is little or no doubt but they 

 are the homologues of the nasal floor cartilages of the higher mammals, though in some 

 respects they differ from those of any higher form. In the typical mammalian condition 

 we have the inner part of the nasal floor cartilage quite simple and curving up by the 

 base of the septum ; in Ornithorhynchus alone among mammals the nasal floor cartilage 

 at its inner part is thickened and excavated to receive the anterior part of the organ of 

 Jacobson. 



The organ itself differs from that of any other known mammal in being made up of 

 an anterior and a posterior part with the opening near the middle, as was shown by 

 Symington. In fig. 1, Plate I., we have represented a transverse vertical section through 

 the anterior part of the organ. The large organ (J.O.) is seen completely surrounded 

 by the nasal floor cartilage, and almost completely divided into an upper and a lower 

 part by a large turbinal plate (t.J.c). The inner part of the nasal floor cartilage rests 

 on the anterior part of the prevomer or dumb-bell shaped bone (Pvo), and at the outer 

 side of the nasal passage the cartilage is seen united with the alinasal (a.n.). In this 

 section is also seen the hinder part of the nasal valve (val.). 



On approaching the region of the naso-palatine canal and the opening of the organ, 

 the turbinal plate becomes considerably thickened, while the cartilages surrounding the 

 two organs become separated by the body of the prevomer. On reaching the plane 

 passing through the opening of the organ, as seen in fig. 2, the cartilage is found to be 

 open below, and the outer part of the cartilage surrounding the organ is seen to be free 

 from the nasal floor cartilage. The organ is here made up of an upper part lying above 

 the turbinal plate and an inner part. Inferiorly, the inner part may be said to open 



