HarleYj on Cutaneous Respiration. 
155 
as having a remarkable parallelism with that already given 
of the cavities in the skin of the frog. In the leaf the cavity 
is situated in the parenchyma (which is analogous to the cutis 
vera of animals) , is covered over by the cuticle (which is equal 
to the animal epidermis) ^ and has an opening in its centre. 
The shape of the aperture varies in different plants^ but it 
is for the most part oval_, as in the skin of the lizard. Some 
botanists adhere to the opinion that the stomates of plants 
are secreting organs^ and not mere passages for the trans- 
mission of gaseous fluids ; but by far the greater number are 
agreed in regarding them as respiratory organs. I am very 
much disposed to ascribe a similar function to the organs 
in the frog^s skin, which resemble so closely the stomates 
in structure. They are exceedingly numerous, amounting 
to many thousands in a square inch of skin (I counted 
about sixty in the field of the microscope with a magnify- 
ing power of 70 diameters), and are found distributed over 
all parts of the body, being most numerous, however, on 
the back and on the dorsum of the limbs ; exactly in 
those positions where the sudoriferous glands in other 
animals are most sparingly distributed. If a comparison 
be instituted between the cutaneous follicles of the frog 
and the true sweat-glands of other animals, a very marked 
difl:erence will be found both as regards their position 
and their structure. Let us for a moment contrast the 
sudoriferous follicles of the human skin, as they are most 
generally known, w ith the organs which have just been de- 
scribed as existing in the integuments of the frog. In the 
palms of the hands and soles of the feet, as well as in the 
axillse and groins of man, the greatest number of perspi- 
ratory glands are aggregated ; and if the cutaneous follicles 
of the frog have a similar function to perform, we might not 
unnaturally expect to find them most numerous in the same 
positions. Such, however, is not found to be the case ; for 
while, in the frog, the back and dorsum of the limbs and other 
exposed positions are thickly studded over with the mouths 
of the cutaneous follicles, the axillae, palms of the hands, and 
soles of the feet, are almost entirely devoid of these organs. 
On the other hand, if we examine into the position occupied 
in the cutis vera by the tegumentary glands of the human 
subject and by those of the frog, we shall find, that while the 
former lie in the deepest, the latter are confined to the most 
superficial layer of the derma. The mode, too, in which the 
glands communicate with the exterior of the body, is in each 
case dissimilar ; for while, in the skin of man, the channel of 
communication is a long, tortuous, and narrow duct, in that 
