156 
Harley^ on Cutaneous Respiration. 
of the frog it is a short_, straight^ and wide passage. In the 
human being there is no special covering to the mouth of the 
duct ; in the frog^ the passage is carefully closed over with a 
delicate layer of epithelium. Again^ in the blood-vessels 
supplying the sweat-glands of man^ we can detect nothing 
peculiar^ while in those aggregated in the neighbourhood of 
the cutaneous follicles of the frog the usual function is found 
to be reversed; the arteries carrying forwards venous bloody 
the veins returning that which has undergone the arterializing 
process. It is thus seen that very little analogy can be traced 
between the sudoriferous glands of man and the cutaneous 
follicles of the frog^ either as regards their structure or 
position. HoAv different is the case when we attempt to 
compare the organs described in these pages with the respi- 
ratory apparatus of some of the lower animals. In the 
pulmonate gasteropods_, for example, we have the lungs 
reduced to a simple sac_, which, in many respects, resembles 
the little cavities in the frog^s skin. In this simple form of 
lung, as well as in the stomates of the frog, we find the cavity 
lined by an involution of the epidermis, and its basement 
membrane surrounded by a network of fine capillaries. The 
contractility of both cavities presents us with still another 
point of resemblance. Dr. Ascherson has pointed out that 
the cutaneous follicles of the frog contract when a stimulus 
is applied to their nerves. He supposed that this property 
was dependent upon a layer of elastic fibres assisting to form 
the wall of the cavity, for at that time he was unacquainted 
with the true structure of the corium. I have not been able 
to detect any contractile fibres in the walls of the follicles, 
which appear to me to consist simply of a basement membrane 
and a layer of epithelium ; but I have distinctly traced the 
widened-out ends of the perpendicular fibres already spoken 
of, ramifying on the exterior of the capsules."^ The contrac 
tion of the cavities, I imagine, therefore, is produced by the 
shortening of the parallel and perpendicular bands of smooth 
muscular fibre composing the greater part of the true skin. 
I must now hasten to bring some evidence of the functional 
importance of the tegument ary respiration. 
It was before mentioned that all animals respire more or 
less by their skin. In 1844, M. Fourcault f drew the atten- 
tion of the French Academy to the interesting fact, that a 
* Hensclie also describes the smooth muscular fibres as composing part 
of the walls of the follicles ; but, as has just been said, they do not, properly 
speaking, belong to the walls of the cavities, for they are only the spread-out 
terminations of the perpendicular fibres. 
t Carpenter's 'Physiology,' p. 702. 
