THE ORGAN OF JACOBSON IN THE DOG. 309 



boundary layer of cells corresponds to the similar layer of 

 inverted conical cells present in the epithelium of the lateral [^ 

 wall, and in the epithelium of the olfactory regiou in some 

 instances. 



On the free surface of the sensory epithelium we notice in 

 sections of hardened specimens numerous fine hair-like pro- 

 jections which are, however, very different from the cilia of 

 the epithelial cells of the lateral epithelium, inasmuch as 

 they are not so long and not so definite hairs as the cilia, 

 but rather represent in their totality an indistinctly striated 

 substance. Whether these striae are prolongations of 

 certain cells only, e.g. the outer processes of the sensortj 

 cells, or whether they are merely prolongations of all cells 

 indiscriminately, i.e. epithelial cells and sensory cells, due 

 to the action of the reagents used, I have not been able 

 definitely to ascertain. 



As regards the extent of the sensory epithelium in the 

 median wall, it differs slightly in the two sides at the same 

 point of the nasal septum, but especially is this the case in 

 different places of the same side. The following measure- 

 ments illustrate this : 



In a transverse section, such as is represented in fig. 9, the 

 extent of the sensory epithelium in the median wall is about 

 0-32 mm., about one fifth of the whole circumference. 



In a section such as is represented in fig. 11 the extent 

 of the sensory epithelium on one side is 0'73 mm., of that 

 of the other side 62 mm. 



In a section represented in fig. VZ the extent of the sen- 

 sory epithelium is about 0-5, in fig. 13 about 0-38 mm. As 

 a rule, the extent of the sensory epithelium in the median 

 wall is greatest in the middle parts of the organ, amounting 

 to about one third of the whole circumference j it diminishes 

 anteriorly and posteriorly. 



The thickness of the sensory epithelium varies between 

 00576 mm. and O'l mm. 



(h) Underneath the sensory epithelium is a fibrous mem- 

 brane containing in some few places lymphoid cells. As a 

 rule there are here numerous venous vessels running longi- 

 tudinally, as well as branches of olfactory nerves ; these 

 form a plexus under the epithelium, and ultimately ascend 

 in an oblique or vertical manner into the sensory epithelium. 

 In fig. 17 these ascending branches are well seen. 



The most important points in connection with the median 

 wall is naturally the presence of numerous branches of 

 olfactory nerves, and their relation to the sensory epithe- 

 lium. In these respects I have found exactly the same con- 



