It may interest you to see these blood-disks in their proper situation, and to observe the motion which they possess during the life of their owners. It is, indeed, one of the most instructive modes of using this wonder-working instrument to look through it at living structures, and watch thet differen processes of life as they are carried on under our eyes. Nor is this at all difficult to accomplish ; for a large number of animals are so small that we can •easily put them upon the stage of the microscope ; and are withal so transparent that their coverings and various tissues offer little or no impediment to our discerning the forms and movements of the contained viscera. And in •cases where the entire animal is too large to be viewed under the microscope as a whole, it sometimes happens that, by a little contrivance, we can so secure the creature as to look without interruptionon certain parts of the body which afford the requisite minuteness and transparency.

I have here a living Frog. You perceive that the web which connects the toes is exceedingly thin and translucent, yet arteries and veins meander through its delicate tissues, which are then clothed on both surfaces with the common skin. But you ask how we can induce the Frog to be so polite as to hold his paw up and keep it steady for our scientific investigation. We will manage that without difficulty.

Most microscopes are furnished (among their accessory apparatus) with what is called a frog-plate, provided for this very demonstration. Here is mine. It is a thin plate of brass, two inches and a half broad and seven long, with a number of small holes pierced through it along the margins, and a large orifice near one end, which is covered with a plate of glass. This is to be Froggy's bed during the operation, for we must make him as comfortable as circumstances will admit.

Well, then, we take this strip of linen, damp it, and proceed to wrap up our unconscious subject. When we have passed two or three folds round him, we bind a tape round the whole, with just sufficient tightness to keep him from struggling. One hind-leg must project from the linen, and we now pass a needle of thread twice or thrice through the drapery and round the small of this free leg, so as to prevent him from retracting it.

Here then he lies, swathed like a mummy, with one little cold foot protruded. Lay him carefully on the brass plate, so that the webbed toes shall stretch across the glass. Now, then, we pass another tape through the marginal holes, and over the body, to bind it to the brass ; of course taking care not to cut the animal, but only using just as much force as is needful to prevent his wrigglings. Now a bit of thread round each toe, with which we tie it to as many of the holes, so as to expand the web across the glass. A drop of cold water now upon the swathes to keep him cool, and a touch of the same with a feather upon the toes to prevent them from drying (which must be repeated at intervals during the examination)— and he is read v.

What a striking spectacle is now presented to us, as with a power of 300 diameters we gaze on the web of the foot ! There is an area of clear colourless tissue filling the held, marked all over with delicate angular lines, some-thing like scales ; this is the tesselated epithelium of the surface. Our attention is caught by a number of black spots, often taking fantastic forms, but generally somewhat star-like ; these are pigment cells, on which the colour of the animal's skin is dej)endent. But the most prominent feature is the blood. Wide rivers, with tortuous course roll across the area, with many smaller streams meandering among them ; some pursuing an independent course below the larger, and others branchingout of them, or joining them at different angles. The larger rivers are of a deep orange-red hue, the smaller faintly tinged with reddish-yellow. In some of these channels the stream rolls with a majestic evenness ; in others it shoots along with headlong impetuosity ; and in some it is almost, or even quite, stagnant. By looking with a steady gaze, we see that in all cases the stream is made up of a multitude of thin reddish disks, of exactly the same dimensions and appearance as those we saw just now in the Frog's blood; only that here, being in motion, we see very distinctly, as they are rolled over each other, that they are disks, and not spherules ; for they forcibly remind us of counters, such as are used for play, supposing they were made out of pale red glass.

It is charming to watch one of these streams: selecting one of medium size, where the density is not too great to see the individual disks, and, tixing our eye on the point where a branch issues from one side of the channel, mark the disks shoot by one after another, some pursuing the main course, and others turning aside into the branch, perhaps so small as to allow of only a single disk to pass at once.

The streams do not pursue the same uniform direction.

The larger ones do indeed ; and their course is from th extremity of the toes towards the body : these are the veins ; but the smaller streamlets flow in any direction, and frequently send out side-branches, which presently return into the stream from which they issued, or unite with others in a very irregular net-work. These are the capillaries which feed the veins, and which are themselves fed by the arteries, who»e course is in the opposite direction, viz., from the body. These, however, are with difficulty seen : they are more deeply seated in the tissues, and are less spread over the webs, being generally placed along the borders of the toes : they are, moreover, fewer

CIECULATION IN FROG S FOOT. and smaller than the veins ; but the blood in them usually flows with more impetuous rapidity.

The variations in the impetus of the current which we observe in the same vessel are probably owing to the mental emotions of the animal ; alarm at its unusual position, and at the confinement which it feels when it endeavours to move, may suspend the action of the heart, and thus cause an interruption in the flow ; or analogous emotions may quicken the pulse. We will, however, now release •our little prisoner, who, though glad to be at liberty, is. as you see, none the worse for his temporary imprisonment.

Let us now look at the circulation of the blood in one of the Invertebrate Animals. In this thin glass cell of sea-

water is a small fragment of sea-weed, and attached to one of its slender filaments you may see three or four tiny knobs of jelly, clustered together like a bunch of grapes. These are animals ; each endowed with a distinct life, but associated together by a common stalk, which maintains the mutual vital connexion of the whole. It is one of the Social Tunicata, and is named Perophora Listeri.

Though each globose knob is no larger than a small pin's head, it is full of organs which carry on the various functions of life ; and, because the whole tissues are as transparent as crystal, they allow us to watch the processes with perfect ease. Take a peep at it.

It is a gelatinous sac, of a form intermediate between globular and cubical, flattened on two opposite sides, with a sort of wart at the summit and another at the side, each of which is pierced with a pursed orifice. The upper of these orifices admits water for respiration and food ; the latter passes through a digestive system, and is discharged through the side PEROPHOKA. orifice. The digestive organs lie on that flattened side which is farthest from your eye, and are therefore dimly seen.

The globose body is enclosed in a coating of loose shapeless jelly, that passes off from one of the lower corners .and forms a short foot-stalk, which unites with similar

foot-stalks from the sister globules, and all together are attached to the sea-weed. Each foot-stalk has an organic core, into which a vessel passes from the body.

Your attention is first arrested by the breathing sac, with its rows of oblong cells, all in wheel-like motion. It is indeed a wonderful object ; but for the present neglect this, as we will return to it shortly, and direct your consideration to the course of the blood.

It is true the fluid which I so name is not red, like that of the Frog which you have just been gazing at, nor does it carry disks of the same elegantly regular form. But you have the advantage here of tracing, at one view, the whole course of the circulation, from its first rush out of the heart, to its return into that organ again.

At the bottom of the interior, below the breathing sac, there is an oblong cavity, through whose centre there runs a long transparent vessel, formed of a delicate membrane, the appearance of which resembles that of a long bag, pointed (but not closed) at either end, and then twisted in some unintelligible manner, so as to make three turns. This is the heart ; and within it are seen many minute colourless globules, floating freely in a subtle fluid : this is the nourishing juice of the body, which we may, without much violence, designate the blood. Now see the circulation of this fluid. The membranous bag gives a spasmodic contraction at one end, and drives forward the globules contained there ; the contraction in an instant passes onward along the three twists of the heart (the part behind expanding immediately as the action passes on), and the globules are forcibly expelled through the narrow but open extremity. Meanwhile, globules from around the other end have rushed in as soon as that part resumed its usual width, which in turn are driven forward by a periodic repetition of the contraction and expansion of the heart.

The globules thus periodically driven forth from the heart, now let us watch, and see what becomes of them.

They do not appear to pass into any defined system of vessels that we may call arteries, but to find their way through the interstices of the various organs in the general cavity of the body.

The greater number of globules pass immediately from the heart through a vessel into the short foot-stalk, where they accumulate in a large reservoir; but the rest pass up along the side of the body, which (in the aspect in which we are looking at it) is the right. As they proceed (by jerks, of course, impelled by the contractions of the heart), some find their way into the space between the breathing surfaces, through narrow slits along the edges of the sac and wind along between the oval ciliary wheels, which we will presently consider. Besides these, however, other globules wind along between the outer surfaces of the sac and the inner surface of the body-walls.

But to return to the current which passes up the right side : arriving at the upper angle of the body, the stream turns off to the left abruptly, principally passing along a fold or groove in the exterior of the breathing sac until it reaches the left side, down which it passes, and along the bottom, until it arrives at the entrance of the heart, and rushes in to fill the vacuum produced by the expansion of its walls after the periodic contraction. This is the perfect circle ; but the minor streams that had forked off sideways in the course, as those within the sac for example, find their way to the entrance of the heart by shorter and more irregular courses.

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