THE SKIN. 
341 
furnished by a minute capillary network of blood-vessels arising 
from arterial trunks, which bring the blood to the gland to be 
purified, and terminating in venous trunks, which carry off the 
blood when that process has been performed. 
These glands are consequently to be regarded as true excretory 
organs, removing from the blood materials that are no longer 
wanted, and which, if retained, would be injurious. Their size 
varies in different situations, being in the palm of the hand 
from l-100th to l-200th of an inch in diameter, but in the arm- 
pits, where they are largest, and form a very thick layer, they 
reach the size of l-60th of an inch. Their ducts are composed of 
basement membrane and epithelium only ; the latter being an 
inflection from the scarf skin which runsdownthe wall of the duct. 
The length of the tube which constitutes both gland and duct is 
about a quarter of an inch. It is straight while passing through 
the true skin; but becomes strongly spiral while traversing 
the scarf-skin, the turns being as close and regular as those of 
a screw. The diameter of this tube is about 1 -1 700th of an inch. 
We can have little idea of the importance of these little ducts to 
the system from considering any single one of them ; but when 
we come to consider them collectively, we may in some degree 
estimate their value, and the necessity of maintaining their 
functions in healthy action. Over 3,500 of these httle ducts 
have been found to exist in a single square inch of the skin of the 
palm ; and accordingly, taking the length of each at a quarter of 
an inch, as we mentioned above, we find that their aggregate 
length amounts to 73 feet. On a squai’e inch of the heel the 
length would be about 47 feet. About 60 feet would represent 
the average length of these ducts for a single square inch of 
skin for the whole body ; and as the number of square inches 
in a person of ordinary size is about 2,500, we arrive by com- 
putation at the startling’ result, that the aggregate length of 
the sudoriferous ducts of the body is about 28 miles. 
It was to this glandular system we referred, when we said 
there was a beautiful contrivance for regulating the internal 
temperature of the body ; for the perspiration so poured out is 
vaporised principally by the heat of the body ; and in thus 
turning into vapour it renders latent, as all liquids do in under- 
going that change, an enormous amount of heat, which is thus 
being constantly carried away from the body as fast as it 
is generated by the chemical processes constantly going on 
within the system. Hence we see the cause of that burning- 
heat of skin which is so marked a symptom of some diseases 
when the perspiration is completely arrested, causing that 
peculiar harsh, dry skin, which is so well known to the physician 
as the concomitant of this burning heat. 
It is due to the same cause that the blood never exceeds about 
