THE SKIN. 
337 
are at first nucleated cells, and tlie colouring matter of the skin 
resides in the nuclei, and these granules it was that were for- 
merly described as a separate layer under the name of tlm rete 
mucosum. They are very minute, being about l-3000th 
of an inch in diameter at first ; being renewed from below 
as the flattened scales are removed above, they gradually ap- 
proach the surface, and as they do so they more and more lose 
the granular form and assume the scaly character, their diameter 
increasing accordingly to about 1 -600th of an inch. In many 
animals, however, they are much larger than this, for the scales 
of reptiles and fish are indeed only a modified form of these 
epidermal scales in man; and in some of these creatures, as 
serpents, the epidermis, instead of being in a constant state of 
renewal and repair, as in man, is only removed at one particular 
season, when it comes off en masse, and is called the slough of 
the reptile. As may be supposed, the body of the animal is very 
tender after this process, and it goes off and hides itself for a 
season, until nature has repaired the loss of the old epidermis 
by a new one. Something similar to the gradual hardening of 
its new skin which then takes place we see in ourselves in the 
gradual hardening and thickening of a new nail, if we acci- 
dentally lose one, which we may observe to grow in thickness 
as well as in length. In fact, the nails are nothing more or 
less than modified cuticle placed in the position we find 
them in order to give protection and support to the ends 
of the fingers, and so enable the tips of the fingers, which 
are the tactile organs in man, duly to appreciate the nature of 
the bodies with which they come in contact ; and it is found 
that the tactile sensibility of the finger is much impaired by 
the loss of the nail. In some of the lower animals the nails 
are further modified into claws, so as to become weapons of 
defence and offence. 
Into the epidermis or cuticle no nerves or blood-vessels 
penetrate, and it is nourished merely by the transudation of 
the serum of the blood through the walls of the vessels of the 
true skin and subcutaneous areolar tissue ; and as it has no 
nerves it is not itself sensitive, but on the contrary, serves to 
blunt the too exquisite sensation of the true skin. That it has 
no sensibility of its own may be proved when a small portion 
of it is detached from the underlying surface of the true skin, 
as by a blister ; and this is the best way of demonstrating the 
cuticle in a living person, as it is extremely difficult to detach 
any portion of it by mechanical means. 
Next we come to the structure of the cutis, or true skin, 
which is much more highly organized, and consists of two 
kinds of tissue, namely, white and yellow fibres ; the former 
being’ denser and more resisting, and being therefore present 
