of which is composed of a single row of the primitive particle (Fig. 325). The diameter of the fibres varies greatly in different

FIG. 325.

kinds of Vertebrated animals. Its average is greater in Reptiles and Fishes than in Birds and Mammals, and its extremes also are wider; thus its dimensions vary in the Frog from 1-100th to 1-1000th of an inch, and in the Skate from 1-65th to 1-300th; whilst in the Human subject, the average is about 1.400th of an inch,

and the extremes about 1-200th and 1-600th.

h

FIG. 326.

a

426. When the fibrillæ are separately examined, under a magnifying power of from 250 to 400 diameters, they are seen to present a cylindrical or slightly beaded form; and their linearly aggregated particles then appear to be minute cells. We observe the same alternation of light and dark spaces, as when the fibrilla are united into fibres or into small bundles; but it may be distinctly seen, that each light space is divided by a transverse line; and that there is a pellucid border at the sides of the dark spaces, as well as between their contiguous extremities (Fig. 326). This pellucid border seems to be the cellwall; the dark space enclosed by it (which is usually bright in the centre) being the cavity of the cell, which is filled with a highly refracting substance. When the fibril is in a state of relaxation, as seen at a, the diameter of the cells is greatest in the longitudinal direction: but when it is contracted, the fibril increases in diameter as it diminishes in length; so that the transverse diameter of each cell becomes equal to the longitudinal diameter, as seen at b; or even exceeds it. Thus the act of Muscular contraction seems to consist in a change of form in the cells of the fibril in a state of ultimate fibrillæ, consequent upon a contraction between the walls of their two extremities; and it is interesting to observe, how very closely it thus corresponds with the contraction of certain Vegetable tissues, of which the component cells are capable of producing movements, when they are irritated, by means of a similar change of form. The diameter of the ultimate fibrille will of course be subject to variations, in accordance with their contracted or relaxed condition; but it seems to be otherwise tolerably uniform in different animals, being for

Structure of the

ultimate Fibrilla

of Striated Muscu

lar fibre-a,

a

ordinary relaxa

tion; b, a fibril in a state of partial contraction.

the most part about 1-10,000th of an inch. It has been observed, however, as high as 1-5000th of an inch, and as low as 1-20,000th, even when the fibre was not put upon the stretch.

427. The "smooth" or non-striated form of Muscular fibre, which is especially found in the walls of the stomach, intestines, bladder, and other similar parts, is composed of flattened bands whose diameter is usually between 2-2000th and 1-3000th of an inch; and these bands are collected into fasciculi, which do not lie parallel with each other, but which cross and interlace. By macerating a portion of such muscular substance, however, in dilute nitric acid (about one part of ordinary acid to three parts of water) for two or three days, it is found that the bands just mentioned may be easily separated into elongated fusiform cells, not unlike woody fibre in shape; each distinguished for the most part by the presence of a long staff-shaped nucleus, brought into view by the action of acetic acid. These cells, in which the distinction between cell-wall and cell-contents can by no means be clearly seen, are composed of a soft yellow substance, often containing small pale granules, and sometimes yellow globules of fatty matter. In the coats of the bloodvessels are found cells having the same general characters, but shorter and wider in form; and although some of these approach very closely in their general appearance to epithelium-cells, yet they seem to have quite a different nature, being distinguished by their contractile

endowments.

428. In the examination of Muscular Tissue, a small portion may be cut out with the curved scissors; and this should be torn up into its component fibres, and these, if possible, should be separated into their fibrillæ, by dissection with a pair of needles, under the simple microscope. The general character of the striated fibre are admirably shown in the large fibres of the Frog; and by selecting a portion in which these fibres spread themselves out to unite with an aponeurotic expansion, they may often be found so well displayed in a single layer, as not only to exhibit all their characters without any dissection, but also to show their mode of connection with the simple fibrous tissue of which the aponeurosis is formed. As the ordinary characters of the fibre are but little altered by boiling, this process may be had recourse to for their more ready separation, especially in the case of the tongue. The separation of the fibres into their fibrillæ is only likely to be accomplished, in the higher Vertebrata, by repeated attempts, of which the greater number are likely to be unsuccessful; but it may be accomplished with much greater facility in the Eel and other fish, the tenacity of whose muscular tissue is much less. The characters of the fibrillæ are not nearly so well pronounced, however, in the Fish, as in the warm-blooded Vertebrata; and among the latter, the Pig has been found by Mr. Lealand (who has been peculiarly successful in this class of preparations) to yield the best examples.

He lays great stress on the freshness of the specimen, which should be taken from the body as soon as possible after death; and when a successful preparation has been made, it should be preserved in Goadby's solution. The shape of the fibres can only be properly seen in cross sections; and these are best made by drying a piece of muscle, so that very thin slices can be cut with a sharp instrument, which on being moistened again, will resume in great part their original characters. Striated muscular fibres are readily obtainable from the limbs of Crustacea and of Insects; and their presence is also readily distinguishable in the bodies of Worms, even of very low organization; so that it may be regarded as characteristic of the Articulated series generally. On the other hand, the Molluscous classes are for the most part distinguished by the non-striation of their fibre; there are, however, two remarkable exceptions, strongly striated fibre having been found in the Terebratula and other Brachiopods, and also in many Polyzoa. Its presence seems always related to energy and rapidity of movement; whilst the non-striated presents itself, where the movements are slower and feebler in their character.

The

429. Nerve-substance.-Whenever a distinct Nervous system can be made out, it is found to consist of two very different forms of tissue; namely, the vesicular, which are the essential components of the ganglionic centres, and the tubular, of which the connecting trunks consist. The "nerve-vesicles" or "ganglion-globules" are cells, whose typical form may be regarded as globular; but they often present an extension into one or more long processes, which give them a "caudate" or a "stellate" aspect. These processes have been traced into continuity, in some instances, with the axis-cylinders of nerve-tubes; whilst in other cases they seem to inosculate with those of other vesicles. vesicles are filled with a finely-granular substance, which extends into the prolongations; and they also usually contain pigmentgranules, which give them a reddish or yellowish-brown color; but these are commonly absent among the lower animals. It is the presence of this pigment, however, which gives to collections of ganglion-globules in the warm-blooded Vertebrata that peculiar hue, which causes it to be known as the cineritious or gray matter. Each of the nerve-tubes, on the other hand, of which the trunks are composed, consists, in its most completely developed form, of a delicate membranous sheath, within which is a hollow cylinder of a material known as the "white substance of Schwann," whose outer and inner boundaries are marked out by two distinct lines, giving to each margin of the nerve-tube what is described as a "double contour." The centre or axis of the tube is occupied by a transparent substance, which is known as the "axis-cylinder;" and there is reason to believe that this last is the essential component of the nervous fibre, and that the hollow cylinder that surrounds it, serves, like the

tubular sheath, for its complete isolation. The contents of the membranous envelope are very soft, yielding to slight pressure; and they are so quickly altered by the contact of water or of any liquids that are foreign to their nature, that their characters can only be properly judged of when they are quite fresh. Besides the proper tubular fibres, however, there are others, known as "gelatinous," which are considerably smaller than the preceding, and do not exhibit any differentiation of parts. They are flattened, soft, and homogeneous in their appearance, and contain numerous nuclear particles, which are brought into view by acetic acid. They can sometimes be seen to be continuous with the axis-cylinders of the ordinary fibres, and also with the radiating prolongations of the vesicles; so that their nervous character, which has been doubted by some anatomists, seems established beyond doubt. The ultimate distribution of the nerve-fibres may be readily traced in thin vertical sections of the skin, treated with solution of soda. It was formerly supposed that all its papillæ are furnished with nerve-fibres, and minister to sensation; but it is now known that a large proportion (at any rate) of those furnished with loops of blood vessels (Fig. 329, D), being destitute of nerve-fibres, must have for their special office the production of the epidermis; whilst those which, possessing nerve-fibres, have sensory functions, are usually destitute of bloodvessels. The greater part of the interior of each sensory papilla of the skin, is occupied by a peculiar "axile body, which seems to be merely a bundle of ordinary fibrous tissue, whereon the nerve-fibre appears to terminate. The nerve-fibres are more readily seen, however, in the "fungiform" papillæ of the tongue, to each of which several of them proceed; these bodies, which are very transparent, may be well seen by snipping off minute portions of the tongue of the Frog; or by snipping off the papillæ themselves from the surface of the living Human tongue, which can be readily done by a dexterous use of the curved scissors, with no more pain than the prick of a pin would give. The transparency of any of these papillæ is increased, by treating them with a solution of soda.

430. For the sake of obtaining a general acquaintance with the microscopic characters of these principal forms of Nervesubstance, it is best to have recourse to minute nerves and ganglia. The small nerves which are found between the skin and the muscles of the back of the Frog, and which become apparent when the former is being stripped off, are extremely suitable for this purpose; and if they be treated with strong acetic acid, a contraction of their tubes takes place, by which the axis cylinder is forced out from their cut extremities, so as to be made more apparent than it can be in any other way. The "gelatinous" fibres are found in the greatest abundance in the Sympathetic nerves; and their characters may be best studied in the smaller branches of that system. So, for the examination of

the ganglionic vesicles, and of their relation to the nerve-tubes, it is better to take some minute ganglion as a whole (such as one of the Sympathetic ganglia of the frog, mouse, or other small animal), than to dissect the larger ganglionic masses, whose structure can only be successfully studied by such as are proficient in this kind of investigation. The nerves of the orbit of the eye of Fish, with the ophthalmic ganglion and its branches, which may be very readily got at in the Skate, and of which the components may be separated without much difficulty, form one of the most convenient objects for the demonstration of the principal forms of nerve-tissue, and especially for the connection of nerve-fibres and ganglionic corpuscles. No method of preserving the nerve-tissue has yet been devised, which makes it worth while to attempt to mount preparations for the sake of displaying its minute characters; but the general course of the nerve-tubes, and the disposition of the ganglionic vesicles, may be demonstrated in preparations preserved in weak spirit; and when the skin has been injected, the passage of the nerve-fibres to the papillæ can sometimes be traced in vertical sections, mounted as opaque objects, and viewed by reflected light. The following method, recommended by Mr. J. Lockhart Clarke, for the examination of the structure of the Spinal Cord,' would be equally applicable to that of other large ganglionic masses:-A perfectly fresh cord is to be hardened in strong spirit, so that extremely thin sections can be made with a very sharp knife; and such sections, placed on slips of glass, are to be treated with a mixture of one part of acetic acid and three of spirit, which not only makes the fibrous portion more distinct, but also renders the vesicular portion more transparent. If it be desired to preserve such a section, it should be transferred, after maceration for an hour or two in the mixture of acetic acid and spirit, into pure spirit, in which it should be allowed to remain for about the same space of time; from the spirit it should be transferred to oil of turpentine, which soon expels the spirit, and renders the section perfectly transparent, so that it can be examined with high magnifying powers; and it may then be mounted in Canada balsam in the usual manner.

431. Circulation of the Blood.-One of the most interesting spectacles that the microscopist can enjoy, is that which is furnished by the circulation of the blood in the "capillary" bloodvessels, which distribute the fluid through the tissues it nourishes. This, of course, can only be observed in such parts of animal bodies, as are sufficiently thin and transparent to allow of the transmission of light through them without any disturbance of their ordinary structure; and the number of these is very limited. The web of the Frog's foot is perhaps the most suitable for ordinary purposes, more especially since this animal is to be easily obtained in almost every locality; and the following is

See his Memoir on that subject, in "Philos. Transact," 1851.