40 



COMPAEATIVE 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." 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 -^t^ 



Pig. 12. — Stimted ° , j /• j • 



Miisciiiar Fibres, to -^-^ of an luch lu diameter, and lound in 



from the heart of ., , ,tt -t new ^\ 



Man, divided by the ncrve-ceutres (J^ig. 132), the gray por- 

 'Jr^ep^atrSl'! tion of the brain, spinal cord, and other gan- 



cieated portions, gija. Sccoud — the fibvoxis, Consisting of pale, 

 flat, extremely fine filaments. They abound in the sympa- 

 thetic nerves, and are the only nerves found in the Inverte- 

 brates. Thii-d — the tubular. These are much 

 larger than the fibrous, the coarsest being 

 la^oo of ^'1 in«h in diameter. They consist 

 of tubes enclosing a transparent fibre and a 

 fatty substance called the nerve- marrow." 

 The delicate tube itself is called neurilem- 

 ma, analogous to the sarcolemma of mus- 

 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. 13 Structure 



of a Nerve: 1, 

 sheath, or neari- 

 lemmn; 2, med- 

 ullary Bubstance 

 of Schwann ; 3, 

 axis cylinder, or 

 primitive baud. 



