ing cells through fusion of the processes, an arrangement most
beautifully seen in dried bone, because here the cavities and the
canals of the matrix are filled with air. Special modification of
bony tissue, the substance of fish-scales and of the teeth, called
also ivory or dentine, should be mentioned.
Blood and Lymph, here treated in connexion with the connec-
tive substances, are in reality not tissues at all, but nutritive
fluids. Two kinds of nutritive fluids occur in the vertebrates, red
blood and the colorless, weakly opalescent, or cloudy white lymph.
The blood of man and other vertebrates, consists of a fluid and
the organized constituents. The fluid or blood-plasma is, apart
from inorganic constituents, specially rich in proteids; after the
removal of the blood from the blood-vessels a part of these separate
by coagulation and form the blood-clot, made up of fibrin, leaving
a fluid poor in proteids, the blood-serum. The organized con-
stituents, the blood-cells, are distin-
guished as red and white blood-cor-
puscles. The latter, the leucocytes,
are present in smaller numbers and
have great similarity to the amoebae
found in water; they are particles
of protoplasm, contain a nucleus,
devour foreign bodies (for example,
carmine granules injected into the
blood), and move in the ' amoeboid '
manner by putting out pseudopodia
Red Blood-corpuscles. In the
mature condition > the red blood-
corpuscles of vertebrates (fig. 46)
are circular or oval discs, which by external influences (e.g., by
pressure) may temporarily be bent, incised, or otherwise modified
in form, but cannot actively change their shape, because they no
FIG. 45 White blood-corpuscles, o,
of man; b, of the crab (n, the nu-
GENERAL HISTOLOGY, 89
longer consist of protoplasm. Embryologically they arise from
true, nucleated, protoplasmic cells; whether these cells are iden-
Fio. 46. Red blood-corpuscles, a, of man; />, of the camel; c, of the adder; d', of
Proteus (seen from the edge); tl", surface view; e, of a ray; /, of Petromyzon; n,
nucleus (all the blood-corpuscles are magnified 700 times, except d, which is mag-
nified 350 times).
tical with the leucocytes or are special < erythroblasts ' is still
undertermined ; but gradually the protoplasmic cell-body changes
completely into a plasmic product, the stroma of the blood-
corpuscle. If the nucleus be retained in this metamorphosis, there
is a slight swelling in the centre of the disc; if, however, the
nucleus degenerate, the bilateral convexity is replaced by a shallow
concavity. In the latter case, one has, in reality, no right longer
to speak of blood-cells, since all the characteristic constituents of
the cell nucleus and protoplasm have disappeared. Systemati-
cally the red blood-corpuscles are of interest, since non-nucleate
forms are found only in the mammals (fig. 46, a, b), nucleated
ones in all the other vertebrates (c, d). The mammals also have
circular, the other vertebrates oval, discs. To this, however,
exceptions occur, since among the mammals the Typloda (camel,
llama) have oval, the Cyclostomes have circular, blood-corpuscles.
Haemoglobin. The red blood-corpuscles are the cause of the
color of the blood, as well as the agents of one of its most impor-
tant functions, the interchange of gases; both are connected with
the fact that the stroma contains the coloring matter of the blood
or licemoglobin. Haemoglobin belongs to the few crystallizable
proteids and is remarkable for the presence of a small, though
extremely important, quantity of iron, and also for its affinity for
oxygen. Haemoglobin containing oxygen, oxy-haemoglobin, causes
the carmine-like color of the so-called arterial blood; oxygen-free,
' reduced ' haemoglobin causes the dark red, faintly bluish color of
90 GENERAL PRINCIPLES OF ZOOLOGY.
Lymph is distinguished from blood by the entire lack of red
blood-corpuscles and the slight coagulability of its plasma.
Lymph is accordingly a proteid-containing fluid with leucocytes,
which are here called lymph-corpuscles.
In the majority of invertebrated animals there is present only
one kind of nutritive fluid, and not even this in every class; the
fluid is called blood, although it is usually colorless. Where
color is present, it is generally, if not always, a yellowish red or
an intense red; this may, even as in the vertebrates, be caused by
haemoglobin (among the molluscs in Planorbis, Area tetragona,
A. now, Solen legumen, Tellina planata, Pectunculus glycimeris,
and others; among the annelids in the Capitellidae, Glycera,
Poly cirrus, Leprcea, leeches, and earthworms; among insects in
Chironomus). Often other coloring matter occurs instead of
haemoglobin : in the cuttlefish, many snails, and in the lobster and
Limulus, the oxygen is taken up by the bluish haemocyanin, which
contains a trace of copper; in the Sipunculids by haemoerythrin,
etc. The blood-plasma, as a rule, is the seat of the color (Chiro-
nomus, Hirudinea, earthworms, and most other annelids); only
exceptionally do colored blood-corpuscles occur, as in the case of
Area, Solen, and the other mussels mentioned above, and also in
the genus Phoronis. Colored elements containing haemoglobin,
identical with blood-corpuscles, are found besides in the ccelomic
fluid of many annelids (Capitellidae, Glycera, Leprea, Polycirrus),
and in the ambulacral vessels of echinoderms (Ophiactis virens,
some Holothurians). Most widely distributed in the invertebrate
animals are the leucocytes, which are distinguished by their active
amoeboid movements; still even these may be absent, and then the
blood is a fluid without any organized corpuscles.
3. Muscular Tissue.
Characteristics of Muscular Tissue. Most sharply character-
ized functionally is the muscle- tissue, inasmuch as it is the agent
of active movements in the animal body. Since active mobility
occurs in protoplasm, it is important to notice the differences
between the two kinds of movement. The distinctions lie in
the direction and in the intensity of the movement. A mass of
protoplasm has the capacity to move hither and thither in all
directions, because in it there is a high degree of mobility be-
tween the smallest particles. Muscles and hence their separate
GENERAL HISTOLOGY. 91
elements, the muscle-fibres and muscle-fibrils, on the contrary,
can shorten only by correspondingly increasing in
diameter (fig. 47); they can therefore accomplish
motion only in a definite direction, that of the axis
of the muscle. The muscle-substance consequently
is more limited in its movement than is protoplasm,
but on the other hand it has the advantages of
greater energy and greater rapidity. An observer
conversant with the different kinds of motion is
able to decide with considerable accuracy, from the
intensity and rapidity, whether in a given case a
movement has been brought about by the agency of
protoplasm or by the contractile substance in the
narrower sense (muscle-substance). tracted state.
Formation of Muscle-substance. These physiological con-
siderations show that protoplasm and the contractile substance are
morphologically different, and that therefore one must distinguish
sharply between formative cells, or muscle-corpuscles, and the
product of these cells, the contractile substance, just as in the
case of connective tissue, between the connective-tissue corpuscles
and the connective-tissue fibrils. This distinction actually occurs,
but optically it is not equally demonstrable, for the reason that it
is not prominent histologically. In animal histology there are
recognized two kinds, it might even be said two stages, in the
formation of muscle-substance, the homogeneous, or smooth, and
the cross-striated. Since the former looks very similar to non-
granular protoplasm, the boundary-line between it and the
muscle-corpuscle is more difficult to recognize than in the case of
the cross-striated muscle-substance, which in its minute structure
is quite different in appearance from protoplasm. In cross-striated
muscles the contractile portion consists of two substances regularly
alternating with one another in the direction of the contraction of
the muscle, of which the one is doubly, the other singly, refractive
(figs. 24, 47, 50).
Smooth and Cross-striated Muscle-fibres. The smooth muscle-
substance represents a lower stage of development than the cross-
striated, since it chiefly occurs in the less highly organized and
more inactive animals. Interesting in this respect is the fact that
in the two stages of development of one and the same animal the
simple and inert polyp has smooth muscles, while the more highly
organized and actively motile medusa has cross-striated muscles
(fig. 48). The difference in their action has led in the vertebrates
92 GENERAL PRINCIPLES OF ZOOLOGY.
to a peculiar distribution of the muscle-substance, the smooth
musculature being chiefly distributed to the internal organs,
which are not under control of the will (involuntary muscles),
while the musculature of the body, subject to the will and hence
demanding more rapid action, is cross-striated (voluntary muscles).
We must not conclude that the difference between smooth and
cross-striated musculature coincides with the distinction between
visceral and body musculature; it should be noticed that the body
musculature of all molluscs is smooth, the visceral as well as the
FIG. 48. Epithelial muscle-cells, o, of a medusa; b, of an actinian.
body muscles of many insects and Crustacea, and the muscles of
the heart of vertebrates are cross-striated.
It was pointed out above, in connexion with epithelia and
connective tissue, that these tissues differed fundamentally. This
contrast has its bearing in dealing with the muscles, for both
epithelial and mesenchymatous cells may form contractile sub-
stances and therefore there are two genetically different kinds of
muscles, the epithelial and the mesenchymatous (contractile fibre-
cell). Both kinds of muscle-cells can a priori form smooth as
well as cross-striated muscle-substance; but the collection of con-
nective (mesenchymatous) tissue around inner organs has caused
most contractile fibre-cells to be smooth, while most of the
epithelial muscle-cells are cross-striated.
Epithelial muscle-cells are cells of which one end extends to the
surface of the body or the surface of an internal cavity (body
cavity, lumen of the gut, etc.), and may here have a cuticle, cilia,
or flagella, while at the opposite end it has secreted contractile
substance in the form of muscle-fibrils (fig. 48). They combine
the double function of epithelial and muscle cells.
Contractile fibre-cells, on the other hand, are connective-tissue
cells, which usually have surrounded themselves with a layer of
contractile substance; corresponding to their origin, they have the
form of connective-tissue cells, and are spindle-formed or
branched; the branches arising from the ends of the cells (fig. 49).
The similarity of form renders the distinction between ordinary
connective-tissue cells and fibre-cells difficult; if the contractile
layer on the surface be slightly developed, the distinction is im-
possible. To recognize the character of the elements, therefore,
we must choose well-defined examples, in which the uninucleated
or the multinucleated mass, the < axial substance/ is sharply
marked off from the muscle-mass, the ' cortical layer' (fig. 49,
c, d, e).
FIG. 49. Contractile fibre-cells, a, of man; 1)-e, of Beroe (a Ctenophore); ft, young
fibres ; c, branched ends ; d, middle portion of a fibre; e, cross- section.
FIG. 50. Cross-striated primary bundle. (After Gegenbaur.) w, nuclei ; s, a point
where the sarcolemma is plainly shown on account of the tearing of the fibres.
In vertebrates and arthropods the contractile fibre-cells occur
in the vegetative organs as elements of the ' organic musculature ' ;
on the other hand we find here the epithelial musculature in the
cross-striated primary bundles, separated from the epithelium,
and only developmentally referable to the epithelium of the body
cavity (fig. 50). A primary bundle is a cylindrical mass, bounded
externally by a structureless envelope, the sarcolemma. Its con-
tents consist of fine fibrils, which, closely parallel to one another
and pressed closely together, run from one end of the mass to the
94: GENERAL PRINCIPLES OF ZOOLOGY.
other. Each fibril is formed of singly and doubly refractive
parts, which alternate with one another in more or less compli-
cated arrangement. Since now the doubly refracting parts of the
fibrils within a bundle lie at about the same level, there is caused
a cross-striation extending through the whole bundle. Finally,
scattered here and there between the muscle-fibrils are the muscle-
corpuscles, spindle-shaped protoplasmic bodies with a nucleus, the
remnants of the cells which have formed the musculature.
4. Nervous Tissue.
Function of Nervous Tissue. As the muscular tissue brings
about motion, so the nervous tissue serves for the transmission of
stimuli. It communicates the stimulations of the sense-organs at
the periphery to the central nervous system, the seat of conscious-
ness, and here brings about perception (centripetal nerve tracts);
further, it transmits the voluntary impulses to the periphery, par-
ticularly to the musculature (centrifugal nerve tracts). By the
nervous system, finally, the stimuli arising in various places are
co-ordinated, thus furnishing the elements for that which we call
independent psychic activity.
Elements of Nervous Tissue. The agent of the transmission
of stimuli is undoubtedly a specific nerve-substance different from
protoplasm. Hence we speak of nerve fibrillae as of muscle fibrillae,
the product of the special nerve-cells, but the relations involved
are not sufficiently understood.
The elements of the nervous system are divided into ganglion
cells and nerve-fibres, but it must be remembered that these are
not independent of each other, but that the fibres are enormously
elongated processes of the ganglion cells. In the vertebrates the
ganglion cells vary greatly in size; besides small elements there
are large cells, only exceeded by the eggs in size, which correspond-
ingly have large nuclei recalling the germinal vesicles. Unipolar,
bipolar, and multipolar ganglion cells are recognized, the differ-
ences depending upon the number of processes (nerve-fibres) which
arise. In multipolar cells the number is very large (fig. 51) and
are of two kinds, dendrites and axons or neurites. Dendrites are
so called because they branch again and again, not far from their
origin from the cell. The axons (of which there is usually but
one to a ganglion cell) can be followed to a long distance with-
out giving off branches, except here and there lateral side twigs
(collaterals) which arise at right angles to the main fibre; they
often pass over into peripheral nerves,
so the morphological distinc-
tion from dendrites lies in the
greater distance of the region
of branching from the body
of the ganglion cell. In bi-
polar ganglion cells both pro-
cesses are neu rites, the cell
itself thus being an element
intercalated in the course of
a nerve-fibre, as also is a uni-
polar ganglion cell. The single
process of this divides near
the cell in a T-shaped man-
ner, so that the unipolar cell
is to be regarded as a bipolar
ganglion cell in which the two
neurites are united for a short
This conception is intel-
ligible in the light of recent
researches on the structure of
the ganglion cell and its pro-
cesses (fig. 52). Both consist
They branch at their tips,
FIG. 51. Multipolar ganglion cell of man,
(After Gegenbaur.) a, axon.
FIG. 52. Motor ganglion cell from the thoracic region of the spinal cord of a dog,
(After Bethe.) n, nucleus.
of extremely fine fibrillae, and inter- and perifibrillar substances
cementing them together. Each process brings a bundle of
GENERAL PRINCIPLES OF ZOOLOGY.
fibrillae to the ganglion cell, in which they spread out and pass
over into other processes. The branching of neurites and
dendrites is a separation of the contained fibrillse; the ganglion
cell, the place of exchange of fibrillae between the various processes.
Hence the ganglion cell is not a simple cell, but a cell plus plasma
The similar fibrillar structure of nerve-fibres has long been
known. In the central nervous system of vertebrates the most
minute elements are the nerve fibrillae, distinguished from muscle
fibrillae. by the absence of cross-
striation ; from connective - tissue
fibrillae by the ease with which they
are injured; in preserved material
they frequently swell and show vari-
cosities (fig. 53). Many fibrillae
united in a bundle form a nerve-
fibre (fig. 54, A) which is called a
gray nerve-fibre in distinction from
the white or medullated fibres. In
the latter the fibre or axis-cylinder
is surrounded by a medullary sheath
(fig. 54, B) composed of my elm, a fat-
like substance, blackened by osmic
acid and separated into variously
shaped ( myelin drops. ' The medul-
lary sheath appears to act as an
FIG. 53. FIG. 64. F.G. 55. insulator.
FIG. 53. Nerve flbriiise with varicosi- Both medullated and non-med-
ullated fibres can be enclosed in a
'sheath of Schwann.' This is a
feature of the fibres composing the
peripheral nervous system and is lacking in brain and spinal cord.
It is a delicate envelope with nuclei here and there (fig. 55). At
times it forms constructions which cut through the medullary
sheath to the axis-cylinder (nodes of Ranvier).
Multipolar and bipolar ganglion cells also occur in the inverte-
brates, most commonly in the coelenterates (fig. 56), more rarely in
worms (e.g., Lumbricus), arthropods, and molluscs, and then
chiefly in the peripheral nervous system. In the ganglia (the
nervous centres of the last three groups) the ganglion cell usually
gives rise to a single strong process, which, however, is richly pro-
Tided with lateral branches or dendrites (fig. 74). The medullary
ties. (From Hatsohek.)
FiG.54. Non-medullated (_ Q _.. Q fiv ,^
FIG. 55.-Medullated j- nerve-fibres,
A, without, B, with sheath of
Schwann. (From Hatschek.)
GENERAL HISTOLOGY. . 97
sheath and sheath of Schwann are usually absent in invertebrates
even in the peripheral nerves. A thin myelin layer has been rarely
observed in arthropods and annelids. On the other hand the true
conducting elements, the nerve fibrillae, have been seen in inverte-
FIG. 56. Ganglion cells of an actinian.
brate nerve-fibres, and these have been followed into the ganglion
cell in which the afferent and efferent fibrillae are united in a
SUMMARY OF HISTOLOGICAL FACTS.
Cells. 1. The most important morphological element of all
tissues is the cell.
2. The cell is a mass of protoplasm which contains one or
several nuclei (uninucleated, multinucleated cells).
3. The nucleus probably determines the specific character of
the cell, since it influences its functions; accordingly it is also the
bearer of heredity.
4. Cells and nuclei increase exclusively by division or budding.
Tissues. 5. Tissues are complexes of numerous similar his-
tologically differentiated cells.
6. Histological differentiation rests in part upon the fact that
the cells take on a definite form and arrangement, in part upon the
formation of plasmic products, which determine the character of
the tissue (muscle-fibres, connective-tissue fibrils).
98 GENERAL PRINCIPLES OF ZOOLOGY.
Classification of Tissues. 7. According to function and struc-
ture (1) epithelia, (2) connective tissue, (3) muscular tissue, (4)
nervous tissue are distinguished.
8. The physiological character of epithelia is determined by the
fact that they cover the surfaces of the body, their morphological
character in that they consist of closely compressed cells united
only by a cementing substance.
9. According to their further functional character epithelia
are divided into glandular epithelia (unicellular and multicellular
glands), sensory, germinal, and protective epithelia.
10. According to the structure are distinguished simple (cubi-
cal, cylindrical, squamous epithelia) and stratified epithelia,
ciliated and flagellated epithelia, epithelia with or without cuticle.
11. The physiological characteristic of the connective tissues is
that they fill up spaces between other tissues in the interior of the
12. The morphological distinction depends upon the presence
of the intercellular substance.
13. According to the quantity and the structure of the inter-
cellular substance the connective substances are divided into (1)
cellular (scanty intercellular substance); (2) homogeneous; (3)
fibrous connective tissue; (4) cartilage; (5) bone.
14. The physiological character of muscular tissue is its
increased capacity for contraction.
15. The morphological characteristic is the fact that the cells
have secreted muscle-substance.
16. According to the nature of the muscle-substance are dis-
tinguished smooth and cross-striated muscle-fibres.
17. According to the character and origin of the cells (muscle-
corpuscles) the muscles are divided into epithelial (epithelial
muscle-cells, primary bundles) and connective-tisue muscle-cells
18. The physiological distinction of nervous tissue rests upon
the transmission of sensory stimuli and voluntary impulses, and
upon the co-ordination of these into unified psychic activity.
19. The conduction takes place by means of nerve-fibres (non-
medullated and medullated fibrils and bundles of fibrils); the
co-ordination of stimuli by means of ganglion-cells (bipolar,
20. Blood and lymph are proteid-containing fluids; rarely
without cells, they may contain only colorless amoeboid cells (white
GENERAL ORGANOLOG7. 99
blood-corpuscles, leucocytes), or in addition to these also red
21. Red blood-corpuscles occur, in the main, only in verte-
brates and cause the redness of the blood ; they are absent in most
22. When invertebrate animals have colored blood (red,
yellow), this is usually due to the color of the blood-plasma.
23. The red blood-corpuscles are nonnucleated in mammals,
nucleated in all the other vertebrates.
III. THE COMBINATION OF TISSUES INTO ORGANS.
An Organ Defined. Organs are formed from the tissues. An
organ is a tissue complex, marked off from the other tissues, which
has taken on a definite form for carrying on a special function.
Thus a single muscle is an organ which consists of a certain
amount of muscular tissue; with scalpel and scissors it can be
removed from its environment as a connected whole and can still
accomplish a definite movement.
Principal and Accessory Tissues. In each organ there is a
tissue which determines the function of the organ, and therefore
its physiological character. This may be called the principal
tissue, for there may be other accessory tissues present, which
merely support or render possible the function of the principal
tissue. In the muscle of the vertebrates we find, besides the
muscle-fibres, connective tissue which, like a kind of cement,
unites the bundles of muscle; blood-vessels which provide nourish-
ment; finally, nerves by which the muscles are aroused to action.
In the human liver also, besides the functionally most important
part, the liver-cells, blood-vessels, nervous and connective tissues
are present. These accessory tissues are usually found only in the
highly developed organs; in the case of the lower animals they
may be absent ; thus the digestive tract of ccelenterates has only
an epithelial lining; their nervous system consists merely of a
cord of nerve-fibres and ganglion-cells.
Effect of Use and Disuse. It is of the greatest importance for
the permanency of an organ that it be constantly in function.
Living substance is distinguished from the non-living by the fact
that, if it be destroyed by use, it is immediately replaced, often by
more than sufficient to make good the loss. Functioning tissues
and organs under favorable conditions increase in volume; on the
100 GENERAL PRINCIPLES OF ZOOLOGY.
other hand, functionless parts undergo a gradual decrease, which
finally leads to their disappearance.
Change of Function of Organs. The two factors mentioned,
that the permanence of the tissues depends upon continued use,