Time with life commences the
constant process of decay and renewal all over the body.
[Illustration: Fig. 49.]
The liquid portion of the blood consists of material formed from food,
air, and water. From this material the cells of the blood are formed:
first, the white cells, which are incomplete in formation; and then
the red cells, which are completed by the addition of the oxygen
received from air in the lungs. Fig. 49 represents part of a magnified
blood-vessel, _a_, _a_, in which the round cells are the white, and the
oblong the red cells, floating in the blood. Surrounding the blood-
vessels are the cells forming the adjacent membrane, _bb_, each having a
nucleus in its centre.
Cells have different powers of selecting and secreting diverse materials
from the blood. Thus, some secrete bile to carry to the liver, others
secrete saliva for the mouth, others take up the tears, and still
others take material for the brain, muscles, and all other organs.
Cells also have a converting power, of taking one kind of matter from
the blood, and changing it to another kind. They are minute chemical
laboratories all over the body, changing materials of one kind to
another form in which they can be made useful.
Pages:
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148