THE ORGAN OF JACOBSON IN THE RABBIT. 558 
becomes much thickened, while the median labium is prolonged 
in an upward direction. The thickening of the extremity of the 
lateral labium is soon again lost, but the median labium, to the 
hind end of the organ of Jacobson, goes on steadily increasing 
in height, the lateral labium in its height remaiming tolerably 
stationary. The greater part of the organ of Jacobson is sur- 
rounded by the cartilage of this nature, 7. e. a trough- or hook- 
shaped plate, of which the median labium is much higher than 
the lateral one, the former resting with its upper extremity alone 
against the thickened lower margin of the septal cartilage. The 
cartilage becomes, at the same time, thinner as it is traced back- 
wards. Figures 5 and 6 show these points very clearly. All 
the figures having been drawn with the camera lucida the rela- 
tions of size, shape, and position are perfectly exact. 
From the relative length of the median labium of Jacobson’s 
cartilage alone it is easily possible to decide which of two sec- 
tions is more anterior, viz. the one whose median labium is 
shorter. 
As the hind extremity of the organ of Jacobson is approached 
this important change takes place, viz. the lateral labium turns 
inwards with its upper extremity, as if to close against the 
median labium, and the part of this latter that extends above 
this line sooner or later becomes discontinuous from the rest 
(see figs. 8 and 9), and, gradually becoming shorter, altogether 
disappears, so that at the very extremity of Jacobson’s organ, 
viz. when the organ of Jacobson has dwindled down to an ex- 
ceedingly fine tube, the cartilage of Jacobson appears in a 
transverse section of an annular shape, open inwards and out- 
wards. 
The cartilage of Jacobson extends a very short distance beyond 
the tube of Jacobson, and it is then only with the median wall, 
which, however, soon altogether disappears. 
Comparing, then, Jacobson’s cartilage in the rabbit with that 
of the guinea-pig, as described in my former papers, we see this 
remarkable difference, that in the rabbit the cartilage extends as 
far as the organ of Jacobson, and even beyond it, while in the 
guinea-pig a considerable posterior portion of the organ of 
Jacobson is without any cartilage, but is surrounded entirely 
by the bone of the crista nasalis of the superior maxilla (see 
Plate XVII, fig. 7, of this Journal, April, 1881). 
The anterior extremity of Jacobson’s cartilage is in both ani- 
mals very different, as is noticed on a comparison of figs. 3, 4, 5, 
6, of my paper in this year’s April number of this Journal with 
the figures of the present memoir. 
In the guinea-pig Jacobson’s cartilage having formed a com- 
plete capsule, going backwards hecomes again incomplete, the 
