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- 



IG. 12. -striated * t{n S of leated cells, varying from ^Vrr 

 Mnscuinr Fibres, to -j^-g- of an inch in diameter, and found in 



from the heart of ' 



Man, divided by the nerve-centres (Fig. 132), the gray por- 



^ tne brain, spinal cord, and other gan- 

 cieated portions. g}j a> g econ d tliefibrous, 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 

 -oW f 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 sarcolernma of mus- Fl j ^ " 

 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. 



sheath, or neuri- 

 lemma; 2, med- 

 ullary substance 

 of Schwanu ; 3, 

 axis cylinder, or 

 primitive band. 



