148 ANIMAL PHYSIOLOGY. 
been aptly compared to an internal world for the tissues, an- 
swering to the external world for the organism as a whole. 
This fluid is the great storehouse containing all that the most 
exacting cell can demand; and, further, is the temporary 
receptacle of all the waste that the most busy cell requires to 
discharge. Should such a life-stream cease to flow, the whole 
vital machinery must stop—death must ensue. 
Comparative.—It will prove more scientific and generally sat- 
isfactory to regard the blood as a tissue having a fluid and 
flowing matrix, in which float cellular elements or corpuscles— 
a view of the subject that is less startling when it is remem- 
bered that the greater part of the protoplasm which makes up 
the other tissues of the body is of a semifluid consistence. In 
all animals possessing blood, the matrix is a clear, usually more 
or less colored fluid. Among invertebrates the color may be 
pronounced: thus, in cephalopods and some crustaceans it is 
blue, but in most groups of animals and all vertebrates the 
matrix is either colorless or more commonly of some slight 
tinge of yellow. Invertebrates with few exceptions possess 
only colorless corpuscles, but all vertebrates have colored cells 
which invariably outnumber the other variety, and display 
forms and sizes which 
are sufficiently constant 
to be characteristic. In 
all groups below mam- 
. mals the colored corpus- 
cles are oval, mostly bi- 
convex, and nucleated 
during all periods of the 
animal’s existence; in 
mammals they are cir- 
cular biconcave disks 
(except in the #amel 
tribe, the ¢orpuscles of 
which are oval), and in 
post-embryonic life with-" 
Fic. 143.—Leucocytes of human blood, showing amce- out a nucleus ; nor do 
boid movements (Landois). These movementsare they possess a cell-wall. 
not normally in the blood-vessels so marked as pic- é . 
puree here, so that the figure represents an ex- The red cells vary in size 
in different groups and 
sub-groups of animals, being smaller the higher the place the 
animal occupies, as a general rule: thus, they are very large 
in vertebrates below mammals, in some cases being almost 
