8 ANIMAL PHYSIOLOGY. 
class of bodies known as proteids—that is, it consists chemically 
of carbon, hydrogen, a little sulphur, oxygen, and nitrogen, ar- 
ranged into a very complex and unstable molecule. This very 
instability seems to explain at once its adaptability for the 
manifestation of its nature as living matter, and at the same 
time the readiness with which it is modified by many circum- 
stances , so that it is possible to understand that life demands 
an incessant adaptation of internal to external conditions. 
It seems highly probable that protoplasm is not a single pro- 
teid substance, but a mixture of such; or let us rather say, fur- 
nishes these when chemically examined and therefore dead. 
Very frequently, indeed generally, protoplasm contains other 
substances, as salts, fat, starch, chlorophyl, etc. 
From the fact that the nucleus stains differently from the 
cell contents, we may infer a difference between them, physical 
and especially chemical. It (nucleus) furnishes on analysis nu- 
clein, which contains the same elements as protoplasm (with the 
exception of sulphur) together with phosphorus. Nuclei have 
great resisting power to ordinary solvents and even the digest- 
ive juices. : 
Inasmuch as all vital phenomena are associated with proto- 
plasm, it has been termed the “physical basis of life” (Hux- 
ley). 
Tissues.—A collection of cells performing a similar physio- 
logical action constitutes a tissue. 
Generally the cells are held together either by others with 
that sole function, or by cement material secreted by. them- 
selves. An organ may consist of one or several tissues. Thus 
the stomach consists of muscular, serous, connective, and gland- 
ular tissues besides those constituting its blood-vessels, lym- 
phatics, and nerves. But all of the cells of each tissue have, 
speaking generally, the same function. The student is referred 
to works on general anatomy and histology for classifications 
and descriptions of the tissues. 
The statements of this chapter will find illustration in the 
pages immediately following, after which we shall return to 
the subject of the cell afresh. 
Summary.—The typical cell consists of a wall, protoplasmic 
contents, and a nucleus. The vegetable cell has a limiting 
membrane of cellulose. Cells undergo differentiation and may 
be united into groups forming tissues which serve one or more 
definite purposes. 
The chemical constitution of protoplasm is highly complex 
