COMPARATIVE ANATOMY OF THE MAMMALIAN ORGAN OF JACOBSON. 245 



occasionally present, it must be admitted to be more normally absent : it is interesting, 

 however, as showing the value of the general anatomical arrangement of parts as a 

 factor in classification, that even when the organ is quite absent (Pteropus, Nyctophilus) 

 the cartilages still follow the same general arrangement as in Miniopterus where the organ 

 is so well developed. 



Though Miniopterus gives us the simplest form of the Eutherian type, and though it 

 lias certain affinities with the Marsupial arrangement, it is further removed from the 

 Marsupial type than is the Marsupial from the Monotreme. 



If a transverse vertical section be made through the posterior part of the papilla an 



appearance will be presented like that shown in fig. 19. The nasal septum is rather 



slender, and from a little below its base on each side there passes out a thin nasal floor 



cartilage, which is present here as a moderately flat plate, curving slightly upwards at 



its inner end. In the middle line, a considerable distance below the base of the septum 



is a small oval cartilage — a development of the prenasal. By the side of the papilla is 



seen the naso-palatine canal passing upwards and inwards, and at its upper part curving 



first outwards, then inwards. The upper portion is considerably dilated, and represents 



really the anterior prolongation of the lower part of the nasal cavity. Kound this 



dilated portion there is a curved cartilaginous support which surrounds it on its outer, 



upper, and inner sides. On tracing the curved cartilage forwards the inner part becomes 



lost, only a small portion of cartilage remaining on the outer side of the canal at its 



upper end. If we trace the curved cartilage backwards we find a most remarkable 



development. Fig. 20 represents a section a very short distance behind fig. 19, and 



here the naso-palatine canal is found freely opening into the nasal cavity. What in fig. 



19 represented the nasal floor cartilage is here divided into two parts, while the outer 



part of the curved roof of the naso-palatine canal is likewise separated from the inner, 



and, furthermore, the inner part of the nasal floor cartilage is found united with the 



inner part of the curved roof of the canal, forming one piece, which is really Jacobson's 



cartilage. Jacobson's cartilage thus consists of an upper moderately flat portion and a 



lower portion, which is markedly concave, and which has in its concavity the anterior 



continuation of the duct of Jacobson's organ. The outer part of the nasal floor cartilage 



proper becomes lost shortly behind this plane, but the outer part of the cartilage which 



supported the canal (o.n.f.c.) becomes well developed. In fig. 21 we have a section a 



little way behind the naso-palatine canal. Jacobson's duct is seen almost surrounded 



by the well-developed Jacobson's cartilage, while what was the outer canal cartilage 



becomes a very well developed nasal floor cartilage. On the inner side of each cartilage 



of Jacobson is seen the anterior part of the pre vomer. In this region Jacobson's duct is 



a pure duct lined with squamous epithelium. Fig. 22 represents a section through the 



most developed part of the organ. The organ is here almost surrounded by a cartila- 



1 ginous capsule ; while the nasal floor cartilage forms a large flat plate which, to some 



extent, passes below Jacobson's cartilage. The broad posterior part of the prevomer is 



i seen stretching from the one cartilage of Jacobson to the other. 



