HOW ANIMALS MOVE. 153 
the ordinary cause of contraction is an influence from the 
brain conveyed by a nerve. The property, however, is 
independent of the nervous system, for it does not cease 
immediately after death. The amount of: force with 
which a muscle contracts depends on the number of its 
fibres; and the amount of shortening, on their length. 
As a rule, muscles are white in cold-blooded animals, 
and red in the warm-blooded. They are white in all the 
Invertebrates, Fishes, Batrachians, and Reptiles, except 
Salmon, Sturgeon, and Shark; and red in Birds and 
Mammals, except in the breast of the common fowl, and 
the like.” 
It is also a rule, with some exceptions, that the volun- 
tary muscles of Vertebrates, and all the muscles of the 
Lobster, Spider, and Insect tribes, are striated ; while the 
involuntary muscles of Vertebrates, and all the muscles 
of Radiates, Worms, and Mollusks, are smooth. All mus- 
cles attached to internal bones, or to a jointed external 
skeleton, are striated. The voluntary muscles are gener- 
ally solid; and the involuntary, hollow.” 
This leads to another classification of muscles: into 
those which are attached to solid parts within the body; 
those which are attached to the skin or its modifications ; 
and those having no attachments, being complete in them- 
selves. The last are hollow or circular muscles, inclosing 
a cavity or space, which they reduce by contraction. Ex- 
amples of such are seen in the heart, blood-vessels, stom- 
ach, iris of the eye, and around the mouth. In the lower 
Invertebrates, the muscular system is a net-work of longi- 
tudinal, transverse, and oblique fibres intimately blended 
with the skin, and not divisible into separate muscles. As 
in the walls of the human stomach, the fibres are usually 
in three distinct layers. This arrangement is exhibited by 
soft-bodied animals, like the Sea-anemone, the Snail, and 
the Earth-worm. Four thousand fibres have been counted 
