460 
RESPIRATION AND EXCRETION 
nective tissue in which fat is deposited. This is known as 
the subcutaneous layer of fat. This layer becomes very 
thick in corpulent people. 
Leather. — The study of the skin which we have just made 
introduces us to some interesting biological problems con¬ 
nected with one of our commonest possessions — shoes. 
The skin of man is built upon the same plan as the skin of the 
animals from which the leather used in shoes, harnesses, grips, 
and gloves is manufactured. This large industry depends 
upon the structure of the skin, especially the arrangement of 
the fibers in the dermis. In the skin of a fish or an alligator, 
the fibers of the dermis run parallel and at right angles in the 
several layers, while in the hairy animals these same fibers do 
not follow any order of arrangement, which results in the 
production of a felt-work of interlacing fibers producing a 
strong skin which resists tearing or flaking. 
Most of the leather that is used in shoes to-day is from 
the hides of cattle. Great numbers of these animals are 
slaughtered each year for food. Their hides must be removed 
with care so as to avoid cutting, as all such injuries destroy 
the fibers of the dermis. These hides are then sent to tanner¬ 
ies where each is made into a certain grade of leather, such as 
sole leather or the leather for the upper part of the shoe. 
This difference in thickness and firmness is produced largely 
by the method employed in tanning. When a vegetable 
tan is used, such as hemlock bark, oak, or sumac, sole leather 
results; and when a mineral tan is employed, leather suit¬ 
able for the upper part of the shoe is secured. 
Two common parasites, the white grub and the southern 
cattle tick, live in the skin and render it unfit for 
leather so that it is to the advantage of cattle breeders to 
keep their cattle free from these parasites. The United 
States Department of Agriculture has greatly assisted cattle 
raisers in exterminating these destructive parasites. The 
effort to exterminate the parasites, especially the southern 
