ANIMAL ELEMENTS AND TISSUES.
The tissues of which animals consist, like those of plants, are
primarily derived from cells; in fact the essential part of the egg or
óvum, from which all perfect animals originate consists at first only of
a simple cell, with its nucleus and nucleolus.
The animal cell-wall differs from that of the vegetable cell in its
softness and delicacy--also in its chemical composition,--the former
consisting of albúminous (_albúmen_, white of egg) matter, while the
latter is composed of cellular or vegetable-cell substance.
There is also a striking difference between vegetable and animal
tissues, in the circumstance that, while the former retain their
cellular condition to a very great extent, the cells of the latter are
frequently so altered by compression and fusion together, or are
obscured by the great development of the cell-contents, that the
cell-form is obliterated, or can only be discovered by the application
of chemical reagents; and in many instances, the relation of the tissues
to the cell can only be discovered by tracing the growth or development
of the latter from its earliest stages. Hence the examination of the
elements and tissues of animals is not well adapted for those who are
unpractised in the use of the microscope; and in treating of them, we
shall simply notice a few which are most easily examined, beginning with
those found in animals belonging to the subkingdom Vertebráta
(_ver´tebra_, a spine-bone).
MAMMÁLIA.--The animals belonging to this class suckle their young; and
their blood-vessels contain red blood.
_Blood._--This blood consists of a yellowish liquid, in which very
numerous red _blood-corpuscles_ or globules (Pl. IX. fig. 1) are
suspended, and to which the red colour is owing. The blood-corpuscles
are not globular, but discoidal, _i. e._ they are circular and
flattened, the sides being slightly sunk in. Their form is best seen as
they roll over on a slide, after the application of a glass cover. The
coloured corpuscles are cells; they appear yellowish red under the
microscope, the deep red colour of the blood depending upon the large
number of them seen at once and crowded together. It need scarcely be
stated that a drop of blood may easily be obtained, by puncturing the
wrist with a clean needle. The blood is contained in the blood-vessels.
These consist of the ar´teries, which convey the blood from the heart;
the veins, which return it to the heart; and a very fine set of
intermediate vessels, called the cap´illaries. If a little water be
added to a drop of blood on a slide, _colourless corpuscles_, rather
larger than the coloured disks, will be seen scattered among the latter.
These are the colourless or lymph-corpuscles of the blood. They are
truly spherical, and granular on the surface.
_Bone._--In examining a transverse section of a bone, one or several
very large cavities will be seen with the naked eye in the centre of the
section; these contain the marrow, or medulla. In the long bones, the
medullary cavity is single, and runs longitudinally down the bone;
whilst in the flat bones the cavities are numerous, forming cancelli.
Under the microscope, thin transverse sections of bone exhibit oval or
rounded holes, or foramina (Pl. IX. fig. 15), which are sections of
canals conveying blood-vessels through the bone; these are the
_Haver´sian canals_. Around the sections of these canals are seen
numerous concentric rings, indicating layers or lamellæ of bony matter.
The substance of bone presents numerous black, somewhat elongated bodies
(fig. 15 _a_), called the _lacúnæ_ (_lacúna_, a little hollow) or
bone-corpuscles, which are however hollow, therefore not truly
corpuscles, as they were formerly considered. Between the adjacent
lacunæ run numerous fine, dark, branched lines, consisting of very
minute canals, or _canalic´uli_. If the section of bone be viewed by
reflected light, the lacunæ and canaliculi will appear white. In the
dried bone they contain air.
The structure of bone is best seen when viewed as a transparent object
in the dry state; for when the section is immersed in liquid, the lacunæ
and canaliculi become filled up.
The size and form of the bone-corpuscles and canaliculi vary in
different animals, so much so that the Class or Order to which an animal
belongs may be determined by reference to these particulars.
Bone consists of earthy salts deposited in a finely granular form
throughout the substance of cartilage. By soaking a piece of bone in
vinegar or other dilute acid, the earthy salts will be dissolved, the
soft cartilage being left. But the structure of cartilage may be best
observed by making thin sections of the gristle covering the ends of
bones. It exhibits a bluish-white basis (Pl. IX. fig. 16), in which are
imbedded numerous cells or _cartilage-corpuscles_, often undergoing
cell-division. In some kinds of cartilage, the basis is composed of
fibres.
_Muscle._--On examining a piece of the red flesh of an animal under a
low power, the mass will exhibit a number of coarse, parallel,
longitudinal, dark lines (Pl. IX. fig. 20), the substance between these
lines being marked with cross or transverse striæ, or lines, and with
fine longitudinal lines. The coarse longitudinal lines indicate the
intervals between bundles of slender fibres, or _fibril´læ_, of which
muscle consists. The fibrillæ (fig. 20 _b_, _c_) are very difficult to
separate; but when perfectly separated, they are seen to be exceedingly
slender, and to consist of alternately light and dark portions in
regular series. When the fibrillæ of the bundles are in close
apposition, as in the natural muscle, the dark portions, being in the
same lines, by their coincidence form the transverse striæ. The bundles
into which they are combined are surrounded by a delicate skin or
membrane, with a little cellular tissue.
The structure of muscle may be observed in a piece of ham which has been
soaked for a day or two in spirit of wine, the mounted needles being
used to pick it to pieces.
The above-mentioned transversely striated muscular fibre is that found
in the voluntary muscles, or those under the influence of the will. But
there are other muscles in animals which are involuntary, or not subject
to the will; in these the fibrillar structure is absent, the muscular
tissue consisting of simple elongated and nucleated cells.
_Cel´lular tissue._--This fills the interstices between the other
tissues and organs of animals, in the same manner that the vegetable
parenchyma does those of plants. It is not, however, composed of cells,
but of very fine, soft, colourless, and wavy fibres (Pl. VIII. fig. 20
_a_), aggregated into bundles, which interlace so as to leave spaces or
aréolæ between them.
The cellular or areolar tissue may be found in a piece of beef or
mutton, in the intervals of the muscular fibres.
_Skin._--The skin is composed of cellular tissue, its outer surface
presenting a number of projecting blunt points, called _papil´læ_. It
contains a large number of blood-vessels; and when the capillaries are
well filled by injection with a coloured composition, it forms a
beautiful microscopic object.
The skin is covered by the epider´mis or cuticle, which consists of
several layers of cells. It is the epidermis which is raised and covers
the bladders formed by the action of a blister applied to the skin.
_Hair._--The hair consists of long solid filaments (Pl. IX. fig. 9), and
not of hollow tubes, as was formerly supposed. It presents varieties of
structure in different animals, which agree generally in animals
belonging to the same Orders.
Hairs are implanted in pits in the skin; each is swollen at the base to
form the bulb, which is seated upon a papilla of the skin, by which it
is formed or secreted. The hair is an epidermic formation, consisting of
epidermic cells more or less flattened and altered in shape by mutual
pressure.
The colour of the hair is usually seated in the outer or cortical
portion of the stem or shaft, and arises from the presence of
aggregations of minute granules of colouring-matter or _pigment_, as the
colouring-matter of animals is called: in the human hair it forms short
longitudinal stripes (fig. 9). In the central pith or medullary portion
of the hair the cellular structure is more open and distinct than in the
cortical portion, in which the cells are so compressed and consolidated
as only to exhibit the cell-structure after treatment with reagents; and
the medullary cells often contain air.
In grey or white hairs, the whiteness depends mainly upon the presence
of air in the cells of the pith. In the gnawing or rodent animals, as
the mouse or the rabbit, the pigment is partially at least situated in
the cells of the medulla.
In the hairs of many animals, the cuticular or surface-cells of the
shaft are distinctly imbricated (fig. 5), and form beautiful microscopic
objects.
The principal interest in the structure of the hair relates to the three
points above mentioned, viz. the position of the pigment, the
arrangement of the cuticular cells or scales, and that of the cells of
the pith.
The pigment is best examined in hairs moistened with a little spirit of
wine, which displaces the air from the cells of the pith, and renders
the hair transparent; a little water should be subsequently added. The
cuticular scales are also well shown by this proceeding. Towards the
root of the hairs in the mouse, they project beyond the margin, giving
it a toothed or dentate appearance; in the hair of the mole, the bat
(fig. 5), or the wolf, this dentation may also be seen. In the hairs of
some of the foreign bats, the scales are whorled, forming very beautiful
objects.
The cells of the pith (Pl. IX. fig. 7) also present interesting
varieties, being sometimes arranged in a single row, at others in two or
more rows (fig. 6). These are best seen in hairs recently immersed in
spirit or in oil of turpentine; for if the hair be too long soaked in
these liquids, the air will be entirely displaced by them. The cells of
the pith appear black by transmitted and white by reflected light, in
the dry hairs, from the presence of air. They may be well examined in
the hair of the mouse (figs. 6 & 7), or in that of the mole. Wool, which
is the hair of the sheep, consists of curled fibres (Pl. IX. fig. 10),
in which the imbricated arrangement of the surface-scales is very
distinctly seen.
In Pl. IX. figs. 10-13 the fibres of wool, flax or woody fibre, cotton,
and silk are represented together, to allow of comparison; for the
microscope is of great assistance in discriminating these substances
when existing in textile fabrics. The fibres of wool (fig. 10) are
distinguished by their solidity, wavyness, and the imbricated scales;
those of flax (fig. 11) by their thick walls, great length, acute ends,
and their knotty appearance at intervals. The fibres of cotton (fig. 12)
are soft, flaccid, flattened, and often twisted; and those of silk (fig.
13) are solid and very slender. By a little chemical testing, the
discrimination is made still more easy; but for an account of this I
must refer to the Dictionary.
BIRDS.--In the Class of Birds, the structure of the _feathers_ deserves
special notice. Feathers are epidermic formations, or consist of
aggregations of epidermic cells, yet so altered by compression and
fusion together that the cell-structure is in most parts difficult to
detect. In a feather three parts are distinguishable,--the transparent
cylindrical quill; its opake continuation, which is more or less
flattened at the sides, forming the shaft; and the vanes or beards,
which arise from the sides of the shaft, consisting of numerous closely
set, parallel, flattened fibres, called the barbs. The structure of the
barbs forms the interesting object to the microscopist. On examining a
piece of the coloured vane of a somewhat large feather (Pl. IX. fig.
14), a row of fine parallel colourless filaments (_pinnæ_) will be
observed, arising from the opposite sides, the filaments of one side
lying obliquely across those arising from the other; and while the
filaments or pinnæ of one side present a row of little teeth (fig. 14
_c_) near their base, those of the opposite side (fig. 14 _b_) are
provided with as many hooks near their apex, which curve over the teeth
to connect the barbs together. This curious arrangement is adapted to
keep the parts of the feather firmly united, and yet to allow of their
play and flexibility. To observe this structure, a portion of a vane
should be soaked in oil of turpentine, and mounted in balsam.
In the downy feathers (Pl. IX. fig. 17) the barbs are not furnished with
the pinnæ, but present simply whorls of minute spines (fig. 18).
The _bones_ of birds present the same general structure as that of
mammals, the lacunæ being, however, more numerous and smaller.
The _blood_ of birds (Pl. IX. fig. 2) differs entirely from that of
mammals, in the red corpuscles being oval instead of circular, and
convex instead of concave; and each contains a distinct oval and
granular nucleus.
REPTILES.--In reptiles, as the frog, toad, or water-lizard (_Tríton_),
the bone-corpuscles or lacunæ are larger and more numerous than in
either of the former classes; and the blood-corpuscles (Pl. IX. fig. 3)
are comparatively very large, oval, rather concave, and contain a large
granular nucleus.
The smooth water-newt or triton, properly called _Lissotríton
punctátus_, is a very interesting animal in a microscopic point of view.
It may be found in most ponds; and if several are removed in a net, and
kept in a large glass jar, with water-plants, they will live for a long
period. In the spring or early summer they will deposit their eggs upon
the aquatic plants, generally on the under surface of a leaf, which they
bend downwards, so as to protect them. The eggs or ova, are about half
the size of a pea, and consist of a sac containing a transparent liquid,
with a yellowish globule within. After a time these eggs will hatch, and
the larvæ or young newts must be removed from the water, otherwise the
parents will devour them.
If one of these larvæ, which resemble little fish in appearance, be
placed with a little water in the “live-box,” and the cap be very gently
pressed down, so as to fix the body of the animal, the circulation of
the blood may be very beautifully seen in either the fringe-like gills,
which are placed on each side of the neck, or in the tail, a low power
being used; at the same time the beautiful stellate pigment-cells of the
skin will be observed. The structure of the rudimentary spinal column,
which runs down the middle of the back, and consists of simple large
cartilage-cells, may also be made out, when the animal is dead, by a
little dissection with the aid of needles.
FISHES.--In the fourth class of vertebrate animals, which consists of
the fishes, we find interesting structures in the blood, the scales, and
the roe. The corpuscles of the blood (Pl. IX. fig. 4) differ from those
of the Mammalia, but agree with those of birds and reptiles, in being
oval instead of round. The scales of fishes (Pl. IX. figs. 22, 23) are
usually rounded or oval, as in most of our freshwater fishes, when they
are called cyc´loid (κὑκλος, circle); but sometimes they are toothed at
one end (fig. 23 _a_), forming cténoid (κτεἱς, a comb) scales, as in the
perch. Most scales exhibit a number of concentric rings, which are the
indications of laminæ; and many of them are lobed at the margin,
sometimes also having radiate furrows. In the centre are often seen
little rounded solid bodies, having somewhat the appearance of cells,
which are very well seen in the scales of the perch; and in some scales
these bodies are arranged in concentric rows throughout the substance,
as in those of the eel or the cod (fig. 24). The substance of which
scales consist is generally cartilaginous; in some of them, however,
true bony matter is present. Fish-scales are contained within the
substance of the skin, and not merely attached to it by one end, as
appears to be the case in many fishes. In most of our common fishes, as
the roach or perch, the scales project beyond the level of the skin; but
the projecting portion is covered by a thin layer of the skin; and when
the scales are scraped off, this layer, with its elegant stellate
pigment-cells, is usually found adherent to it. In some other fishes, as
the cod and eel, the scales are entirely sunk below the surface; and
these are commonly supposed to have no scales. They may, however, be
easily found by dissection, or by drying a piece of the skin under
pressure between two plates of glass, and mounting a portion in balsam.
The beautiful silvery lustre of the skin of fishes depends upon the
presence of innumerable very minute and thin crystals; these may be
well examined in the skin of a sprat.
The roe of fishes consists of the ova or eggs, and the spermatozoa,--the
ova being contained in the hard, the spermatozoa in the soft roe. The
eggs consist of a cell surrounded by one or two membranes; and the
latter are often traversed by numerous fine radial canals, or present a
funnel-shaped tube leading to the ovum. The spermatozoa of the soft roe
consist of exceedingly slender filaments (fig. 25), terminated at one
end by a kind of head. The reader will not fail to detect the analogy
between the ovum of the animal and that of the ovule of the plant; and
it need scarcely be stated that the spermatozoa of the animal fertilize
the ova, in the same manner that the pollen-tubes and spermatozoa of
plants fertilize the ovules existing in them. In the case of fishes, the
spermatozoa of the soft roe escaping into the water, and moved by the
ciliary action of the filament, enter the micropyle-like canals of the
ova, which are deposited by the fish upon the bottom of rivers.
The scales of fishes may be prepared for examination by scraping them
off and macerating them in water until the adherent portion of the skin
is softened and decomposed, so that it may be washed away. They should
be dried between glass plates, and viewed under a low power, as dry
transparent objects.
The structure of muscle can be more easily made out in fishes than in
other animals. A portion of the flesh should be macerated in spirit as
directed above.
MOLLUS´CA.--We shall now leave the vertebrate animals, and pass to the
subkingdom Mollusca, the marine kinds of which are popularly called
shellfish: three of their structures form interesting objects for
examination--the shell, the tongue, and the gills.
_Shell._--The general structure of the shell of the Mollusca may be
illustrated by reference to that of the oyster. Two kinds of
shell-substance are at once distinguishable in an oyster-shell, an outer
brown, and an inner pearly or nacreous. The brown portion exhibits under
the microscope the appearance of a cell-structure (Pl. IX. fig. 28), the
angular forms from mutual pressure being very distinct. The component
bodies of this portion are seen to be more or less elongated and
flattened in the side view, forming prisms (fig. 29). The structure of
the pearly part of the shell is more difficult of examination, and can
only be seen distinctly in ground and polished sections. In these, under
a high power, it exhibits numerous fine, somewhat parallel wavy lines
(fig. 19), which are the indications of thin layers, or laminæ, of which
it is composed.
Shell consists of a basis of animal matter in which carbonate of lime
(chalk) is deposited, the whole being poured out or secreted by the skin
or mantle of the mollusk.
Pearls, which possess the same structure as the nacreous part of shell,
consist of the nacre formed around some foreign body, as a grain of
sand, &c., by which the mantle has been wounded.
_Tongue._--The structure of the tongue of the Mollusca is very
interesting, on account of the curious teeth which are found upon it. It
may be illustrated by the common Whelk (_Buc´cinum unda´tum_), which is
sold at the street-stalls. As, to one unacquainted with the anatomy of
the Mollusca, there is some difficulty in finding the tongue, it may be
well to point out how it is to be found. If the shell containing the
animal be placed so that its orifice is directed upwards, the point or
apex of the spire being towards the reader, the lid (oper´culum) which
closes the shell will be at once evident. On drawing the animal from the
shell by means of the lid, the foot or portion which is applied by the
animal to the surface upon which it creeps will be seen. At the upper
part of this is the head, with its two horns (ten´tacles). Below the
roots or bases of the tentacles, and between them and the upper part of
the foot, is the little round mouth. On slitting this up with scissors,
a cavity will be opened, and in it will be seen a reddish tube (the
proboscis), about as large as a goose-quill, with an aperture at the
end. This must be carefully slit up, when the tongue, which is of about
the size of a crow-quill, will come into view. The tongue is moveable in
the proboscis, and can be protruded or withdrawn by the animal at will.
If the surface of the tongue be viewed under a handlens, the rows of
teeth will be seen at once. It is better not to pull the tongue out with
forceps, as the teeth are easily displaced and injured. The best plan is
to dissect away the muscular structures with forceps and a pair of
fine-pointed scissors, then to cut off the tongue at its root, and to
soak it in water for some hours, when the skin or epidermis containing
the teeth can be separated with the mounted needles under a simple lens
or microscope. After any loose particles have been washed away with a
hair pencil, the object may be spread flat on a slide, and dried between
two slides. The upper slide should then be removed, the tongue soaked in
oil of turpentine, and mounted in balsam with the least possible heat.
As thus prepared, the horny teeth (Pl. IX. fig. 21) are seen to be
arranged in rows, united by a colourless membrane, so as to form a long
ribbon. The teeth form three longitudinal parallel rows, a central and
two lateral. Each tooth, considering the separate pieces as constituting
distinct teeth, has little teeth or denticles at its lower edge. These
are curved inwards, four in number, and connected by a basal plate in
the side teeth; while the middle teeth have six or seven straight
denticles. These teeth serve to enable the animal to scrape or rasp the
algæ, and other matters forming their food, from the surfaces upon
which they grow. And if some water-snails are placed in a glass jar the
inside of which is covered with confervoid growths, the curious patterns
left after the action of the snails’ tongues will be found to present a
very curious appearance.
_Gills._--The gills or “beards” of the oyster or mussel exhibit very
strikingly the phenomenon of ciliary motion. The gills (branchiæ) are
respiratory organs, consisting of folds of the skin, covered with cilia,
by means of which the water in which the animal lives is set in motion,
and constantly changed to aërate the blood within them. The currents
thus induced serve also to bring the food which floats in the water
towards the mouth of the animal. By snipping off a thin portion of one
of the brown beards of a fresh oyster, laying it upon a slide, adding a
drop of the “liquor” contained within the shell, and lightly pressing a
cover upon the whole, the remarkable phenomenon to one who has not
before viewed it will be seen under a somewhat high power--about ¼-inch.
The whole field will appear in motion, and the lashing or whip-like
action of the cilia will be seen, especially towards the edges of the
bars (Pl. IX. fig. 36) of the gills. The rapid motion of any floating
particles present will also be noticed, showing the direction of the
currents of liquid, which, as the liquid is transparent, would not
otherwise be recognizable.
BRYOZÓA (βρὑον, moss, ξῶον, animal).--The animals included in this
Class, which belongs to the Mollusca, are mostly marine. They are
microscopic, and contained in horny or calcareous sacs or cells,
aggregated together to form polyp´idoms (_pol´ype_, and δῶμα, a house).
They are sometimes plant-like or leafy (Pl. IX. fig. 27), at others
filamentous and branched, or they form a layer or crust upon the objects
to which they are attached. The polypidoms, which are often some inches
in length, are frequently met with on the seashore, the cells (fig. 26
_a_) having slit-like valvular orifices. The bodies of the animals are
soft and polype-like, and are furnished at one end with a circle of
tentacles, covered with rows of cilia, by which the water is changed for
respiration, and particles of food are brought to the mouth. The
tentacles can be protruded or withdrawn at the will of the animal. The
Bryozoa are what are called compound animals, each individual body
having its own set of organs; yet the whole are connected together.
The two species figured are very common. _Flus´tra foliácea_ (Pl. IX.
fig. 27) is found everywhere upon the sea-shore. The polypidom has cells
upon both sides; and they are narrowed at one end, and rounded at the
other. _Membranip´ora pilósa_ (Pl. IX. fig. 39) occurs upon sea-weeds
and other marine bodies, forming a closely adherent layer. The orifices
of the cells are surrounded with teeth, and are usually furnished below
with a very long bristle--the polypidom appearing to the naked eye as a
white hairy crust. In the variety figured, the long bristles are
replaced by a spine; and this is not uncommon.
The polypidoms of the Bryozoa form interesting microscopic objects, the
cells being furnished with variously arranged spines and punctures or
dots. In some the cells are erect and arranged in rows upon the branches
of a plant-like stem, while in others they are scattered irregularly
over a creeping filament.
For examination they should be prepared by maceration in fresh water,
and drying between glass plates or sheets of paper, and either viewed as
opake objects or, after soaking in turpentine and mounting in balsam, as
transparent objects.
It may be remarked that the name Bryozoa for this class of Mollusca,
which was thoroughly established, has recently been changed in this
country to Polyzóa (πολὐς, many, ξῶον, animal), and that the name of
polypidom has been altered to polyzóary.