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. 16 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- 

 FIQ 12 stri d s ^ n f nucleated cells, varying from ^\nr 

 Muscular Fibres, to -^^ of an inch in diameter, and found in 



from the heart of , .. . . 



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

 iuto 7ep r arate e nu* ^ on f tne bniin, spinal cord, and other gan- 

 cieated portions. gjj a> Second t\\efibrous, 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 

 -rVb-Tr 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 iwurilem- 

 ma, analogous to the sarcolemma of mus- F 'j 

 cular tissue. Nerve -tubes are found only sheath, or neun- 



lemma; 2, med- 



m back -boned animals, in the white sub- uiiary substance 



,,.,. . i j j . , i of Schwaim ; 3, 



stance of the brain, spinal cord, and in the axis cylinder, or 



nerves. primitive band. 



A bundle of fibrous or tubular nervous matter, sur- 

 rounded by connective tissue, constitutes a nerve. 



