THE ORGAN OF JACOBSON. 



161 



These lateral recurrent cartilages are structurally continuous 

 with the base of the nasal septum and with the anterior nasal- 

 floor cartilages. The other parts of the section are as in that 

 previously described, except that the alinasal is curving down to 

 form the outer nasal wall. 



On passing backwards the nasal septum is found to become 

 completely detached from the anterior nasal-floor cartilages and 

 from the recurrent flaps which remain united to^ form a pair of 

 large recurrent cartilages. This condition is seen in fig. 10. The 

 alinasal curves round to form a complete outer nasal wall. 



Fig. 11, though representing a section only a very short 

 distance behind that of fig. 10, shows the pair of large recurrent 

 cartilages reduced to two pairs of very small structures. As will 

 be seen in the later sections, the upper cartilage is continued 

 backwards to form the upper part of Jacobson's cartilage. The 

 lower cartilage/which lies in the nasal floor, ends abruptly. The 

 section passes through the anterior part of the premaxillary. 



Fig. 12, a short distance behind the section represented in 

 fig. 11, shows a section through the anterior part of the papilla. 

 The upper part of the section is fairly similar to that of the 

 previous section figured, but below the premaxillary is seen the 

 mode of opening of the naso-palatine ducts by the sides of the 

 small papilla. Each duct is supported by a scroll of cartilage 

 completely round it except at the opening, and a few sections 

 further forward show that the cartilage also protects the duct in 

 front. 



On passing backwards the cartilaginous scroll becomes divided 

 into an upper and inner, and a lower and outer part. The 

 former becomes the lower part of Jacobson's cartilage ; the 

 latter the posterior nasal-floor cartilage. In the section repre- 

 sented by fig. 13 the anterior end of Jacobson's organ is seen 

 opening into the naso-palatine duct. 



Fig. 14 represents a section a short distance further back.^ The 

 palatine process of the premaxilla is seen detached. Above it lies 

 the upper part of Jacobson's cartilage. The lower part of 

 Jacobson's cartilage has the typical U-shaped appearance on 

 section. There is still seen a small posterior nasal-floor cartilage. 



Fig. 15 represents a section behind the anterior palatine 

 foramen. The palatine processes of the premaxillse give support 

 to the cartilages of Jacobson. The organ is here well developed, 

 and the cartilage has the form seen in most higher mammals. 

 The nasal-floor cartilage is no longer present, the_ floor being 

 supported by the secondary palatal plates of the maxillary. 



Fig. 16 represents a section far behind that represented by 

 figure 15 and near the posterior end of the organ. The organ 

 is still fairly large and the cartilage still of the typical shape. 

 The palatine processes do not extend so far back, and the car- 

 tilages are now in part supported by the vomer and in part by 

 the maxillaries. 



If the sections of the snout in Gymnura be compared with 



