40 
COMPARATIVE ZOOLOGY. 
branes, which show a nucleus under the microscope, and 
do not break up into fibrils (Fig. 122). The gizzards of 
fowls exhibit this form. 
All muscle has the property of shorten¬ 
ing itself when excited ; but the contraction 
of the striated kind is under the control of 
the will, while the movement of the smooth 
fibres is involuntary. 18 Muscles are well sup¬ 
plied with arteries, veins, and nerves; but 
the color is due to a peculiar pigment, not 
to the blood. 
Muscular tissue is found in all animals 
from the Coral to Man. 
(8) Nervous Tissue. —Nervous matter exists 
under three forms: First—the cellular , con¬ 
sisting of nucleated cells, varying from 
of an inch in diameter, and found in 
the nerve-centres (Fig. 132), the gray por- 
imo separated tion of the brain, spinal cord, and other gan- 
cieated portions. Second—th q fibrous, consisting of pale, 
flat, extremely fine filaments. They abound in the sympa¬ 
thetic nerves, and are the only nerves found in the Inverte¬ 
brates. Third—the tubular. These are much 
larger than the fibrous, the coarsest being 
-ranj- of an inch in diameter. They consist 
of tubes enclosing a transparent fibre and a 
fatty substance called the nerve-marrow. 17 
The delicate tube itself is called neurilem¬ 
ma, analogous to the sarcolemma of mus- Fl ° f Nerve^ 0 ? 
cular tissue. Nerve-tubes are found only 
in back-boned animals, in the white sub¬ 
stance of the brain, spinal cord, and in the 
nerves. 
A bundle of fibrous or tubular nervous matter, sur¬ 
rounded by connective tissue, constitutes a nerve. 
Fig. 12. — Striated 
Muscular Fibres, to - 5 -Jfr 
from the heart of 
Man, divided by 
sheath, or neuri¬ 
lemma; 2 , med¬ 
ullary substance 
of Schwann; 3, 
axis cylinder, or 
primitive band. 
