40 COMPARATIVE ZOOLOGY. 
Muscular tissue is found in all animals, from the Coral 
to Man. 
(8) Nervous Tissue-——Nervous matter exists under three 
forms: First—the cellular, consisting of nucleated cells, 
varying from gjy5o to gdp of an inch in 
diameter, and distributed through the body, 
particularly in the gray portion of the brain, 
spinal cord, and nerve-centres, or ganglia. 
Second—the jibrous, cousisting of pale, flat, 
extremely fine filaments. They abound in 
Fia. 12.—Structure Os ae OKT 
of a Nerve: 1, the sympathetic nerves, the only nerves 
sheath, or neuri- found in the Invertebrates. Third —the 
lemma; 2, med- 
ullary substance tubular. ‘These are much larger than the 
of Schwann; 3, . 1 : 
axis cylinder, or fibrous, the coarsest being yoo of an inch 
mitive band. =. : . 
pomeve "in diameter. They consist of tubes inclos- 
ing a transparent fibre and a viscid fluid called the nerve- 
marrow.” The delicate tube itself is called newrilemma, 
Fic. 13.—A Ganglion of the Sympathetic Nerve of a Mouse. 
analogous to the sarcolemma of muscular tissue. Nerve- 
tubes are found only in back-boned animals, and chiefly in 
the white substance of the brain and spinal cord. 
A bundle of fibrous or tubular nervous matter, surround- 
ed by connective tissue, constitutes a nerve. 
