The transition from the solid to the tubular form of hair is beautifully seen in the hairs of certain animals, as illustrated in the accompanying drawing of sections.

A. Hairs from the ant-eater, showing the central part filled with pith.
B. Hair of peccary, with greater development of the pith.

c. Section of hair of walrus, indicating the commencement of the tubular
form.-Magnified 100 diameters.

D. Section of the whisker of the tiger, in which the tube is perfect.
E. Section of hair of an elephant, showing the combination of a number
of tubes united together, exactly resembling the arrangement of the
hoof-horn, another cutaneous appendage.-Magnified 50 diameters.

Hairs and hair-like bodies will often occur as accidental objects upon the slide, and may give rise to either serious or ludicrous mistakes, in proof of which it is only necessary to allude to a recent instance of a hair from a rat having been sent for examination, as a new form of entozoon found in the secretion from the ear.

For obvious reasons the objects most commonly present accidentally, are fibres of wool from flannel, and fibres of cotton, linen, or silk, with the characters of which the student should be familiar, in order to avoid confounding them with any other structures under examination. Illustrations of these objects are given, as a guide to assist the observer in detecting them when present.

A. Linen fibres, solid, round, and tapering, not twisted, and possessing no
medulla.

B. Cotton fibres, flattened and twisted tubes, having an indistinct medulla.
c. Sheep's wool, distinguished by the covering of imbricated scales and
generally the absence of medulla.

D. Silk fibres, round and solid, without any medulla; occasionally a few
twisted fibres are seen, but they are easily distinguished from the
flattened tubes of cotton.

OF THE NAILS.

NAIL is nothing more than epidermis, the elements of which have assumed the character of horn.

A nail is divided into three parts: 1. root; 2. body; 3. edge.

The root is covered by a fold of skin, immediately in front of which is a white portion, resembling a half-moon in shape, and termed the lunula.

Matrix is that part of the cutis vera which secretes the nail. It differs but little from the papillated structure of the corium in other parts, except that under the body of the nail it is very dense, and is arranged to form ridges or lamellæ, which correspond to the under surface of the nail.

Like the epidermis, nail is composed of two layers, an upper hard or horny surface, and a lower softer stratum (stratum Malpighii).

The position of these parts can only be seen in good sections, to make which some care is necessary. First a nail with its bed or matrix must be separated from the bone of the finger, and dried until sufficiently firm to be sliced with the razor.

Transverse and longitudinal sections may be made, and on being placed in a little water on the slide, they soon swell up and resume their natural characters.

Under a low power the appearances shown in the illustration will be recognised, but the minute structure of nail cannot be perfectly seen without the aid of chemical agents.

Transverse section through the body and bed of the nail; magnified 8
times. a. Bed of the nail, with its ridges (black). b. Corium of the
lateral parts of the wall of the nail. c. Stratum Malpighii of the same
part. d. Stratum Malpighii of the nail, with its ridges (white).
e. Horny layer on the wall of the nail. f. Horny layer of the nail, or
proper nail substance, with shallow notches upon its under surface.

H

If a portion of the section should happen to be sufficiently thin,

a high power will show the two layers of which the nail is composed, as in the next drawing.

Fig. 82.

f

C

Transverse section through the body of the nail; magnified 250 diameters. A. Cutis of the bed of the nail. B. Mucous layer of the nail. C. Horny layer of it, or proper nail-substance. a. Laminæ of the bed

B of the nail. b. Laminæ of the stratum Malpighii of the nail. c. Ridges of the nail itself. d. Deepest elongated cells of the mucous layer of the nail. Upper flat-cells of it. f. Nuclei of the proper substance of the nail.

e.

Longitudinal sections are more difficult to make and are not so instructive as transverse ones. The parts seen in the next figure will rarely be obtained perfectly in any one specimen.

Longitudinal section through the middle of the nail and its bed; magni-
fied 8 times. a. Bed of the nail, and cutis of the back and point of the
finger. b. Mucous layer of the point of the finger. c. Of the nail.
d. Of the bottom of the fold of the nail. e. Of the back of the finger.
f. Horny layer of the point of the finger. g. Beginning of it under the
edge of the nail. h. Horny layer of the back of the finger. i. Termi-
nation of it upon the root of the nail. k. Body. 1. Root. m. Free
edge of the proper substance of the nail.

The minute structure of nail, of which very little is usually seen in section, may be studied in specimens that have been soaked for some hours in caustic potass. A scraping of the softened nail should be placed in a drop of water on the slide, teazed out, and covered in the usual way.

Under a high power very delicate nucleated cells will become apparent, as represented in the woodcut.

Fig. 84.
A

B

Nail-plates, boiled with caustic soda; magnified 350 times. A. Viewed
from the side. B. From the surface. a. Membranes of the distended
elements. b. Their nuclei from the surface. c. from the side.

The cells forming the nail may also be rendered apparent by allowing a drop of caustic potass to flow under the edge of the covering glass on to a fine section under examination. By this method the arrangement of the cells in layers will be clearly seen. During the growth of the nail it appears that the soft or mucous layer (with its ridges) remains in its position, while the upper or horny layer is continually pushed forward from the root.

The growth of a nail from root to edge has been found to Occupy twelve weeks.