NERVOUS TISSUE. 9 



The unstriated niuscle is composed of flat, elongate, spindle- 

 shaped bodies, which contract slowly and remain in a con- 

 tracted state for some time. They seldom are more than -g^ 

 of an inch in length. They form muscles over which we 

 have no control, and are thus called involuntary muscles. This 

 variety is prevalent in the lower animals, but is also found in 

 all high forms of life. Each smooth muscle-cell has one distinct 

 nucleus. 



Striated or voluntary muscle consists of multinucleated masses 

 called primitive bundles. It is composed of long cylindrical 

 fibres, about -g-J-g- of an inch in diameter in mammalian muscle. 

 Most or all of the cell protoplasm is converted into a cross- 

 striped substance, due to the alternate double and single re- 

 fractive powers. This striped or voluntary muscle is under the 

 control of the animal will, and can contract with great energy. 

 Almost the entire protoplasmic contents of the cells are con- 

 cerned in the production of this voluntary muscular tissue. The 

 cells become elongated into long fibres, the primitive bundles; 

 and the nucleus divides and forms numbers of nuclei, each 

 fibre being surrounded by a membrane, the so-caUed "sarco- 

 lemma." The sarcolemma is an elastic sheatli. The primitive 

 bundles also arise by the fusion of several cells. Muscular 

 tissue, then, is also ceU-tissue modified for a certain definite 

 object — namely, movement. There is certain striated muscular 

 tissue called cardiac muscle, which forms the walls of the heart, 

 and is involuntary in action. Cardiac muscle is cubical in 

 form, and has a little side projection from each cell. 



4. Nervous Tissue is found generally with muscular tissiie. 

 It forms the seat of will and sensation, and is the means by 

 which stimuli are carried to the muscles to cause their move- 

 ment. The nervous tissue is supposed to have originated from 

 the ectodermal sense-cells found in the skin, and that, still re- 

 maining united to the same, they have grown inwards, and have 

 thus only in a secondary way become united to the muscle-cell, 

 which is prima fa/sie contractile. In nervous tissue there 



