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142 ANATOMY.

excepting at such places as the palmar surface of the hand, soles of the
feet, face, and the calvarium, Avhere it is attached to the fascia beneath
by numerous stout fibrous trabeculse, the spaces between these bands
being filled with cushions of fat.
In the region of the face and neck, as is shown by the action of the
muscles of expression, -the skin is movable and is under the control of
the striated or voluntary muscular structui'e.
The thickness of the skin varies in different regions of the body. On
the back, the palmar and plantar regions it is very thick, while in the
inguinal and axillary regions and on the eyelids it is extremely thin.
Hairs, either long or short, coarse or fine, are found protruding from
the skin throughout almost its entire extent, but they are much more
plentiful in some places than in others.
The Tactile Corpuscles are in the papillse of the skin ; they are the
principal organs of touch, and are capable of a high degree of cultiva-
tion, as is aptly illustrated in the marvellous sensitiveness of many blind
persons. By this sense can be detected degrees of heat and cold, hard-
ness and softness, and the direction of the air-current when but gentle.
The sensitiveness of the skin varies in different parts of the body, it
being most acute at the tips of the fingers and lips.
The skin is also an important excretory organ, and, under certain
circumstances, is capable of effecting absorption, its functions in this
respect varying in different parts of the body.
In some of the lowest orders of animals there is no integumentary
covering, while in others of a somewhat higher scale there is a distinct
outer layerof cells performing the functions of the integument. Others
are provided with special organs of secretion, as is illustrated in the shell
membrane of the mollusca, etc.
In the higher forms of animal life (the vertebrates) the integument
can be separated into two great divisions—the epidcrm, or cuticle, or
scarf skin, and the derm, or corium, or cutis vera—synonymous names
for the skin layers that have much complicated its study.
The Blastoderm at a very early stage of its existence divides into two
layers, the epiblastio and the hypoblastic ; a third, or mesoblastic layer,
derived from the contiguous portion of the epiblast and hypoblast, forms,
and is situated between them. From the epiblastic or upper layer
are formed the epiderm, or cuticle of the skin, and all its appendages,
such as the hair, the nails, the enamel of the teeth, the brain, and the
nerves. From the mesoblastic or middle layer are formed the true
skin, the cartilage, the bones, muscles, the dentine and cementum of the
teeth, etc. From the hypoblastic or lower layer are formed the epithe-
lium of the mucous membrane and the various glands of the alimentary
canal situated posterior to or below the ])alato-glossal fold of the mouth,
and in front or above the lower third of the rectum.
Fig. 71 is a diagram of the skin divided into different strata or layers.
The first natural or embryonic division is formed through its separation
into two layers, the upper one, the epiderm, being derived from the
epiblastic, and the lower one, the true skin, from the mesoblastic layer,
an apparent basement-membrane (hereafter explained) being situated
between them.