166 ANIMAL PHYSIOLOGY. 
probably to take up the work of many other kinds of cells 
when the needs of the economy demand it. The fluid matrix 
or plasma furnishes the lymph by which the tissues are direct- 
ly nourished, and serves as a means of transport for the cells 
of the blood. 
The chemical composition of the blood is highly complex, in 
accordance with the function it discharges as the reservoir 
whence the varied needs of the tissues are supplied; and the 
immediate receptacle (together with the lymph) of the entire 
waste of the body; but the greater number of substances exist 
in very minute quantities. The blood must be maintained of 
a certain composition, varying only within narrow limits, in 
order that neither the other tissues nor itself may suffer. 
The normal disorganization of the blood results in coagula- 
tion, by which a substance, proteid in nature, known as fibrin, 
is formed, the antecedents of which are probably very variable 
throughout the animal kingdom, and are likely so even in the 
same group of animals, under different circumstances; and a 
substance abounding in proteids (as does also plasma), known 
as serum, squeezed from the clot by the contracting fibrin. It 
represents the altered plasma. 
Certain well-known inorganic salts enter into the composi- 
tion of the blood—both plasma and corpuscles—but the princi- 
pal constituent of the red corpuscles is a pigmented, ferrugi- 
nous proteid capable of crystallization, termed hemoglobin. It 
is respiratory in function. 
THE CONTRACTILE TISSUES. 
That contractility, which is a fundamental property in some 
degree of all protoplasm, becoming pronounced and definite, 
giving rise to movements the character of which can be pre- 
dicted with certainty once the form of the tissue is known, finds 
its highest manifestation in muscular tissue. 
Very briefly, this tissue is made up of cells which may be 
either elongated, fusiform, nucleated, finely striated lengthwise, 
but non-striped transversely, united by a homogeneous cement 
substance, the whole constituting non-striped or involuntary 
muscle; or, long nucleated fibers transversely striped, covered 
with an elastic sheath of extreme thinness, bound together 
into small bundles by a delicate connective tissue, these again 
into larger ones, till what is commonly known as a “muscle” 
