In all branches of natural history, but perhaps preeminently in microscopic natural history,—owing to its greater liability to error from deceptive appearances,—we gain much information on any given structure by comparing it with parallel or analogous structures in other forms. Thus we shall find that our understanding of the structure of this hair will be much increased when we have seen, under the same magnifying power, specimens of the hair of other animals. In order, however, to explain it, I must anticipate those observations.

What Ave see, then, is a perfectly translucent cylinder, having a light brown tinge, and marked with a great number of delicate lines, having a general transverse direction, but very irregularly sinuous or winding in their individual courses. These lines we perceive to be on the surface ; because, if we slowly turn the adjustmentscrew, the lines grow dim on the central part of the cylinder, while those parts that lie near the edges (speaking according to the optical appearance) come into distinctness. Presently the edges of the cylinder become sharply defined, and are seen to be cut into exceedingly shallow saw-like teeth, about as far apart as the lines ; b 2 these, however, are so slight that they can be seen only by very delicate adjustment. We go on turning the screw, and presently another series of transverse lines, having the same characters as the former, but differing from them individually, come into view, at the sides first, and presently in the middie, and then, as we still turn, become dim, and the whole is confused. In fact, our eye has travelled, in this process, from the nearer surface of the hair, right through its transparent substance, to the farther surface : and we have seen that it is surrounded by these sinuous lines, which the edges—or those portions of the hair which would be the edges, if it were split through the middle (for, optically, this is the same thing) —show to be successive coats of the surface, suddenlv terminated. If we suppose a cylinder to be formed of very thin paper, rolled up, and then, by means of a turninglathe, this cylinder to be tapered into a very lengthened cone, the whole would be surrounded by lines marking the cut-through edges of the successive layers of paper ; and, owing to the thickness of the paper not being mathematically equal in every part, these edges would be sinuous ; exactly as we see in these lines upon the hair. The effect and the cause are the same in the two cases.

A hair is closely analogous to the stem of a plant : inasmuch as it grows from a root, by continual additions of cells to the lower parts, which, as they lengthen, push forward the ever-extending tip. Indeed, in some of the hairs which we shall presently look at, there is the most curious resemblance to the stem of a palm, with the projections produced by the successive growth and breaking away of leaf-bases around the central cylinder. Internally, too, the resemblance is remarkable ; for, if we split a human hair, and especially if we macerate it, i.e. soften it by soaking it, in weak muriatic acid, we shall find it composed of (1) a thin but dense kind of bark, forming the successive overlapping scales just described :.

{2) a fibrous substance, extending from the bulb to the point of the hair. By soaking the hair in hot sulphuric •acid, this fibrous substance resolves itself into an immense number of very long cells, pointed at each end, and squeezed by mutual pressure into various angular forms. " A human hair, of one-tenth of a line in thickness,* has about 250 fibrils in its mere diameter, and about 50,000 in its entire calibre : so that these ultimate fibrils are finer than those of almost any other known tissue, from the great elongation and narrowing of their constituent cells as they are drawn out into the shaft of the hair during growth : and hence the expanded bulb of the hair, where the cells are yet spherical and soft."f (3) Running through the very centre of the fibrous portion may be sometimes discerned a dark slender line, which is a sort of pith (medulla) composed of minute roundish cells, filled Avith air, and arranged in two or three rows. J

HOG S HRISTLE*.

The bristles of the Hog bear much resemblance to the human hair. On this slide is one (a), which you perceive

This is nearly thrice as great as the diameter I have given above, which i.s the result of several careful admeasurements of diffeient hairs, taken from childhood and adult age.

f Grant, "Outl. Comp. Anat.," 647.

£ " The cortical (or bark-like) substance has different colour, according to the colour of the hair ; generally, the colour is diffused through its whole mass ; less frequently, the colour depends on granular pigment scattered through its substance in small masses. The cortical substance

is just thrice as thick as the hair that we have been examining, or y^^th of an inch in diameter. The sinuous lines across the surface are proportionally far finer and closer together, and no saw-teeth are visible at the edge, the most delicate adjustment showing only a minute undulation in the outline ; that is to say, the overlapping scales are far thinner, and therefore their terminations are nearer together, in the hair of the Swine than in that of Man. I will now show you a transverse section of a similar bristle, which I will obtain thus : I take this old brush, and with a razor cut off one of the bundles of bristles, close to the wood ; then I take off as thin a shaving as I can cut, wood, bristles, and all ; I repeat the same operation two or three times. Now, having picked out the shavings of wood, I take up with the point of my penknife a few of the dust-like atoms that remain, and scatter them on this plate (or slide) of glass, and these I cover with another plate of thin glass; for this dust is composed of thin transverse slices of the bristles, and as I scatter

contains a number of cavities filled with air, most evident in the hair from aged persons, or in dry hair.

" The central portion, the medulla, forms, when well developed, an axis-cylinder, one-fifth, or one-fourth, the diameter of the hair, with sharp outlines, generally central, but often a little excentric in position. It is often wanting in human hair, especially in blond hair. ... In woolly hair it is always wanting ; also in the hair of the new-born child. The medullary substance is often interrupted, and sometimes consists of only a few dark points lying in the axis of the hair . . The medullary substance has been thought to contain the pigment : this is not so, the supposed pigment granules being very minute air- bubbles. The cause of the colour of the hair is found in the diffuse pigmentation of the cortical substance. The cause for the hair becoming grey or white is to be found in the disappearance of the diffuse pigmentation of the cortical substance, the cause of which is not yet known. The medullary substance can be more easily seen in white hair than in coloured." (From a valuable memoir, entitled, " Hair in its Microscopical and Medico-legal Aspects," by Dr. E. Hofman ; transferred to the "English Mechanic" for May 9, 18/3, from the " Xew York Medical Journal.") them, some will fall upon their cut ends, so that we shall look through them endwise.

Here is one, very suitable for examination (b),—since it is not a whole section, the razor having passed somewhat obliquely across it, coming out beyond the middle, where it thins away to an edge. The outline is not circular, but elliptical; that is, the hair is not round, but flattened. There is no separable cortex, or bark, and the whole substance appears as if made up of exceedingly fine fibres, of which we see the ends cut across. A rough dark line occupies the middle of the slice, in the plane of the greater diameter ; but at the edge of the slice we are able to see that this is not a solid core, as has been sometimes supposed, but a cavity passing up through the hair. It is surrounded by a layer of cells, called medullary, and which appear black, because they are filled with air.

The finer hairs of the Horse and the Ass, such as those selected from the cheeks, have the sinuous edges of the plates about as close as in human hair. But they are distinguished at once by the conspicuousness of the medullary portion, which is thick, and quite opaque, and is broken up (especially towards each extremity of the hair) into separate longitudinal irregular masses.

The fine wool of the Sheep is clothed with imbrications, * proportionally much fewer than those of human hair, while the diameter is also much less. Thus these examples, selected from fine flannel and from coarse worsted, vary in diameter from ^^th to T£-^th of an inch ; and there are, upon an average, about two imbrications in a space equal to the dia- FIBRE OF meter. No colour is perceptible in these spe- SHFKP 'S woot"

A structure is said to be imbricated when it is arranged like tiles on the roof of a house.

cimens ; they are as transparent and colourless as glass. The imbricated plates project here considerably more than in either of the examples we before examined ; the " teeth," however, form an obtuse angle.

We shall presently see the importance of this imbricate structure ; but we will first look at a few more examples, in which we shall find it still more strongly developed, in conjunction with some other peculiarities. All the hairs that we have looked at are what I have called fibrous in their interior texture, but those of many animals are more distinctly cellular.

Thus, in these specimens, plucked from the fur of the Cat that lies coiled up on the hearthrug, we see, first, that the imbrications are short, being about equal to thediameterinlength,but are very strongly marked ; though, like those of the Sheep's wool, obtuse. Hence the outline is extremely like that of the stem of an old rough palmtree. There is a distinct bark (cortex), which is thick, and marked with longitudinal lines, which add to the resemblance just alluded to. The interior is clear, marked off at pretty regular intervals by the broad flattened medullary cells, in single series, each cell occupying, for the most part, the whole breadth of the interior. These cells are transparent, and apparently empty ; but their walls appear opaque and almost black,—an optical HAIR OF CAT. illusion, dependent on the absorption of the light by their surfaces at certain angles with the eye of the beholder. The fibrous portion is here almost displaced by the great development of the medullary cells.

In the larger hairs of the Mole, which we will now look at, the bark is very thin ; and though the surface is marked with sinuous lines, these do not project into teeth.

The pith here again forms the greater portion of the hair, the cells of which it is composed being placed in single series, which, for the most part, extend all across the body of the hair, though they are somewhat irregular both in size and shape. They are rather flattened, and appear perfectly black (that is, opaque) by transmitted light, their surfaces absorbing all the rays of light. The small hairs of the same animal, however, are very different in form : they are flattened, so as to appear twice as broad in one aspect as in another at right angles to it ; HAIRS OF MOLE. and, what is curious, the scales of the bark project into strongly-marked imbrications on one side, and are scarcely perceptible on the other. Here, as in the larger hairs, there is a single row of oval transverse cells, perfectly opaque.

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