THE SENSE ORGANS 145 



H 



FIG. 89. A-D, VARIOUS STAGES OF DEVELOPMENT OP [THE SO-CALLED] JACOBSON'S ORGAN 

 OF THE UKODELIA, illustrated by a series of transverse sections. F, transverse 

 section through the nose and Jacobson's organ of Lacerta agilis ; G, the same of 

 a placental Mammal ; H, the same of Ornithorhynchus, after Symington ; I, diagram- 

 matic side view of G. 



In A the organ commences medially and basally ; in D the lateral position is attained ; 

 E, the Gymnophione, in which separation from the principal cavity is effected ; na., 

 nasal cavity ~ f jc., Jacobson's organ ; c.j., Jacobson's cartilage ; g.tn., inter-maxillary 

 gland ; g.n., nasal gland ; n.o., olfactory nerve ; n.t., trigeminal nerve ; d.n., nasal 

 duct ; mx., upper jaw ; sp., septum nasi ; o.d., dumb-bell-shaped bone, forming a 

 support for Jacobson's organ. 



cavity (jc., Fig. 88, G, H, I). In most Mammals this becomes 

 constricted off and secondarily connected with the buccal cavity. 

 A lateral displacement does not take place, and the organ remains 

 between the floor of the nasal cavity and the roof of the mouth, 

 i.e. in its original position. It is always lined with a pronounced 

 sensory epithelium, innervated by the ventral fasciculus of the 

 olfactory nerve (n.o., Fig. I). 



Eecent investigation has proved, without doubt, that vestiges 

 of a Jacobson's organ are to be found in adult human beings. 

 Before considering these in detail, however, certain structures 

 which attracted the attention of the earlier investigators need 

 to be dealt with. 



Huscke's "plough-share cartilage" in Man was formerly 

 regarded as the vestige of the two cartilaginous tubes lying near 

 the base of the nasal septum, which in many lower Mammals 

 envelop the organ of Jacobson. This is incorrect, since, as 

 Spurgat has shown, the same cartilages are found in the human 

 organs of Jacobson as in those of the lower Mammalia, but in a 

 much reduced condition. These organs, together with the Sten- 

 son's canals, open into the buccal cavity through the ductus 



