Hence, I think that, in these slit scales, the upper layer is formed, as commonly believed, by successive deposits from beneath ; but that, after a few have been deposited, they begin to slit, probably by contraction in becoming solid ; that the lower layer is formed after each upper one is hardened, exceeding its length by a little, and filling up the slit ; that this lower layer becomes the upper layer of the next course, slitting, and turning up its terminal edge as it hardens ; that then the lower layer is deposited on this, filling up the slit as before ; and that this process goes on as long as the fish lives.

It is curious that, in the scales of the Pike, the portions thus separated by slitting, instead of expanding and leaving spaces to be filled up, actually close over each other, the divided parts overlapping considerably, as you may see in these specimens. The left-hand scale (a) is from the back ; the central one (6), which has only a deep narrow incision instead of a tube, is from the lateral line ; and the third (c) is from the belly of the fish.

Let us return now to the scales of our Gold-fish, and examine a highly interesting structure connected with them. The brilliant golden or silvery reflection that constitutes the beauty of these lovely fishes, depends not on the scales themselves, but on a soft layer of pigment spread over their inner surface, and seen through theirtranslucent substance. On carefully detaching a scale, we see on the under side, opposite to that portion only which was exposed (all the concealed parts being colourless), a layer of soft gleaming substance, easily separable, either silvery or golden, according to the hue of the fish. If now we remove a small portion of this substance with a fine needle, and

SCALES OF PIKE.

spread it on a plate of thin glass, we shall find, by the aid of the microscope, that it consists of two distinct substances ; the one giving the colour, the other the metallic lustre. Witha power of 300 diameters, the former is seen to be a layer of loose membranous cells, of an orange colour in what are properly called the Gold-fishes, and whitish or pellucid in the Silver-fishes. If we now add a minute drop of water to the mass, and gently agitate it with the point of a needle, and again submit it to the microscope, we shall have a beautiful and interesting spectacle. The water around the mass is seen to be full of an infinite number of flat spicula or crystals, varying much in size, but of very constant form, a flat oblong prism

with angular ends (as represented in the accompanying engraving). By transmitted light they are so transparent and filmy as to be only just discernible ; but by reflected light, and especially under the sun's rays, thev flash like plates of polished steel. But what appears most singular, is that each spiculum is perpetually vibrating and quivering with a motion apparently quite spontaneous, but probably to be referred to slight vibrations of the water in which spicula of gold-fish's they float \ and each independently of

SCALES. the i rest, so as to convey the i 1 impres- • sion to the observer that each is animated with life, though the scale be taken from a lish some days dead. Owing to this irregular motion, and consequent change of position, each spiculum, as it assumes or leaves the reflecting angle, is momentarily brightening or waning, flashing out or retiring into darkness, producing a magic effect on the admiring observer. To this property, I suppose, is to be attributed the beautiful pearly play of light that marks these lovely fishes, as distinguished from the light reflected by an uniformly polished surface. I have found the pearly pigment of the scales to be provided with similar spicula in fishes widely differing in size, structure, and habits ; as the Gudgeon and Minnow, the Pike and the Marine Bream. The spicula of these fishes agree in general form with those of the Gold-fish : and also in size, with the exception of trifling variations in the comparative length and breadth. The colouring matter is lodged in lengthened cylindrical cells, arranged side by side, and running across the scale :. that is, in a direction at right angles to the lateral line..

Chapter II. Blood

The microscope is daily becoming a more and more important aid to legal investigation. An illustration of this occurred not long ago, in which a murder was brought home to the criminal by means of this instrument. Much circumstantial evidence had been adduced against him, among which was the fact, that a knife in his possession was smeared with blood, which had dried both on the blade and on the handle. The prisoner strove to turn aside the force of this circumstance by asserting that he had cut some raw beef with the knife, and had omitted to wipe it.

The knife was submitted to an eminent professor of microscopy, who immediately discovered the following facts :— 1. The stain was certainly blood. 2. It was not the blood of a piece of dead flesh, but that of a living 1 »ody ; for it had coagulated where ic was found. 3. It was not the blood of an ox, sheep or hog. 4. It was human blood. Besides these facts, however, other important ones were revealed by the same mode of investigation. 5. Among the blood were found some vegetable fibres. 6. These were proved to be cotton fibres,—agreeing with those of the murdered man's shirt and neckerchief. 7. There were present also numerous tesselated epithelial cells. In order to understand the meaning and the bearing of this last fact, I must explain that the whole of the internal surface of the body is lined with a delicate membrane (a continuation of the external skin), which dis- •charges mucus, and is hence termed mucous membrane. Now this is composed of loose cells, which very easily separate, called epithelial cells ; they are in fact constantly in process of being detached (in which state they constitute the mucus), and of being replaced from the tissues beneath. Now microscopical anatomists have learned that these epithelial scales or cells, which are so minute as to be undiscernible by the unaided eye, differ in appearance and arrangement in different parts of the body. Thus, those which line the gullet and the lower part of the throat are tesselated, or resemble the stones of a pavement ; those that cover the root of the tongue are arranged in cylinders or tall cones, and are known as columnar; while those that line some of the entrails carry little waving hairs {cilia) at their tips, and are known as ciliated epithelium.

The result of the investigation left no doubt remaining: that with that knife the throat of a living human being, which throat had been protected by some cotton fabric, had been cut. The accumulation of evidence was fatal to the prisoner, who without the microscopic testimony might have escaped.

But what was there in the dried brown stain that determined it to be blood % And, particularly, how was it proved to be not the blood of an ox, as the prisoner averred 1 To these points we will now give a moment's attention.

With this fine needle I make a minute prick through the skin of my hand. A drop of blood oozes out, with which I smear this slip of glass. The slip is now on the stage of the instrument, under a power of 600 diameters. You see an infinite number of small roundish bodies, of a clear yellowish colour, floating in a colourless fluid, but so numerous, that it is only here and there, as near the edges of the smear, that you can detect any interval in their continuity.

These bodies are what we frequently call the bloodglobules, or, more correctly, blood-disks ; since their form is not globular, but thin and flat, like that of a piece of money. The slightness of their colour is dependent on their extreme thinness : when a larger number lie over each other the aggregated colour is very manifest, as it then becomes either full dark red, or bright rich scarlet ; for to these disks blood is entirely indebted for its wellknown hue. The blood of all vertebrate animals is composed principally of these bodies, which when once seen are easily recognised again : the microscope then readily determines whether any given red fluid or dried stain is composed of blood.

The disks in the blood of Mammalia, or animals which suckle their young, are circular or nearly so, and slightly concave on both of the surfaces. On the other hand, in Birds, Fishes, and Reptiles, their form is elliptical, and the surfaces are flat, or slightly convex. This distinction, then, will at once enable us to determine Mammalian blood.* But to determine the various tribes of this great class among themselves, we must have recourse to another •criterion,—that of dimensions.

The blood-disks of Man nearly agree in size with those of the Monkey tribe, of the Seals and Whales, of the Elephant, and of the Kangaroo. Most other quadrupeds have them smaller than in Man ; the smallest of all being found in those animals which chew the cud. The little Musk-deer of Java has disks not more than one-fourth as large as the human, but these are remarkably minute ; no other known animal approaches it in this respect : those of the Ox are about three-fourths, and those of the Sheep little more than half the human average.

Tables have been made out, showing the comparative

The Camels among Mammalia, and the Lampreys among Fishes, are exceptions to the above rule ; the former having ellijotical and convex blood-disks, the latter circular and slightly concave.

size of these corpuscles in various animals, and such tables are very useful ; but we must bear in mind that the average dimensions only are to be looked for ; since in any given quantity of blood under examination, we shall not fail to see that some disks exceed, while others come short of, the dimensions of the majority.

Generally speaking, the blood-disks in Birds and in Fishes are about equal in size : their form is, however, that of a more elongated ellipse in Birds than in Fishes. They may be set down as averaging in breadth the diameter of the human disks, while their length is about half as much again, or a little more, in most Birds.

It is in Reptiles that we meet with the largest disks, and especially in those naked-skinned species, the Frogs and Newts. A large species inhabiting the American lakes—Siren lacertina—has disks of the extraordinary size of l-400th of an inch long by l-800th broad, or about eight times as large as those of Man, in linear measure. Our common Newts afford us the largest examples among British animals, but they do not reach above half the size just mentioned.

Taking this drop of blood from my finger as a standard of comparison, we find, on applying the micrometer, that

the disks rim from l-2500th to 1 -5000th of an inch ; but that the great majority are about l-3300th in diameter. On these slides are samples of other kinds. This is the blood of a Fish—the common Blenny or Shanny (Bleniilus pltolis). Here we see at once the oval form of the disks ; their average is l-2800th by l-3300th of an inch. Here is the blood of a Frog (Rana temporaria) ; these are more than twice the size of the fish's ; for they average l-1250th by 1-1 800th of an inch. And, finally, I can show you a drop of blood from this Smooth-newt (Lissotriton punctatis). The large size of the disks is now plainly seen, and so indeed is the elegance of their form : in this case, as in the last, we see in each disk a distinct roundish nucleus. These run from l-703th to l-950th in length by 1-1 100th to 1-1 600th in breadth; but the average are about 1 -800th by 1-1 300th of an inch.

Page 5 of 66