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Urachus. The bond of union which at an early period connects the urinary bladder with the allantois in the embryo; it is subsequently obliterated in

the fœtus.

Vacuoles. Small cavities occurring in cells. They are supposed to have important functions in the unicellular organisms.

Vagus. The part of the eighth pair of nerves distributed to the viscera of the throat, thorax and abdomen; the great regulating nerve of the vegetative functions. Vaso-constrictor. Those impulses which excite contraction of the vascular muscles.

Vaso-dilator. Those impulses which inhibit the action of the vascular muscles. Vasomotor. Those nervous mechanisms controlling the movements of the blood vessels.

Villus. A hair-like process. A term applied to the small projections characteristic of the small intestine. They contain blood vessels and lacteals, and are important in absorption.

Vitellus. The yolk of the ovum, which in mammals divides completely to form the embryo. In birds only a part divides, and the rest serves to nourish the chick. Vorticella. Bell animalcule, a bell-shaped unicellular organism with a rudimentary, ciliated mouth cavity and rapidly contractile stalk.

Wolffian body. An embryonic structure, the forerunner of certain parts of the genito-urinary apparatus.

Zymogen. A peculiar substance supposed to give rise to the pancreatic ferments.

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Blood, circumstances influencing

coagulation, 248.

clot, 247.

coagulation of the, 245.
coagulation within the ves-
sels, 249.

colorless corpuscles, 225.
colorless corpuscles, origin
of, 227.

composition of, 215.
constitution of the, 214.
corpuscles, 217, 224.

corpuscles, increase in num-
ber of white, 225.

corpuscles, number of, 224.
corpuscles, separation from
plasma, 218.

crenated red corpuscles,
230.

current, velocity of the, 313.
defibrinated, 246.
density of the, 231.
development of red discs,

241.

enumeration of blood cor-
puscles, 233.

fibrin formation in, 252
fibrin of, 219, 222, 246.
fibrinoplastin of, 222.
fibrin in, of, 222.
gases in the, 243, 357.
hæmatin of, 240.
hæmatoidin of, 240.
hæmin of, 240.
hæmochromogen of, 240.
lakey, 232.

methemoglobin of, 239.
of amphioxus, 228.

of cold-blooded animals,
252.

origin of colorless corpus-
cles of, 227.
oxygen in the, 245.
oxyhæmoglobin of, 235.
plasma, coagulation of, 220.
plasma, chemical composi-
tion of, 220.

preparation of oxyhæmo-
globin, 236.

red corpuscles of, alteration
in shape, 230.
red corpuscles, 228.

red corpuscles forming rou-
leaux, 231.

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Carbon dioxide in the blood, 244.
Carbonates in the tissues, 81.
Carbonic acid, 351.

Carbonic acid gas CO, in the
tissues, 80.

Cardiac centre, 643.

cycle, 265.

impulse, 270.

movements, 268.

nerve mechanism, 277.

Cardiograph, 271.
Cartilage. 57.

Casein, 70, 383.
Catelectrotonus, 507.
Cathode, 504.
Cell contents, 34, 36.
Cell sap, 34.
Cell wall, 31, 36.
Cells, animal, 33.

budding of, 86.
development of, 86.
differentiated, 38.
division of, 86.
fat, 37.

ganglion, 49, 521.
germination of, 85.
goblet, 200.
history of, 33.
indifferent, 38.
liver, 34.

of mucous tissue, 53.
nerve, 49, 521.

nucleus and nucleolus of,
34.

pigment, 37.

reproduction of, 85, 87.
varieties of, 38.

vegetable, 33.

Cellular theory, 33.
Cellulose, 91.

Centrifugal nerves, 496.

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