NERVOUS TISSUE. 9 



poN\'ei's. This strijjed or voluntary muscle is under the con- 

 trol 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-called "sarco- 

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

 bundles also arise by the fusion of several cells. Muscular 

 tissue, then, is cell-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 

 which is involuntary in action. Cardiac muscle is cubical in 

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



4. Nervous Tissue is found generally with muscular tissue. 

 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 musole-cell, 

 which is prima facie contractile. In nervous tissue there are 

 two distinct elements — namely, nerve-cells and nerve-filaments — 

 which have separate structural differences. 



Nerve-cells are found in the brain, in the spinal cord, in 

 the so-called ganglia of the lower animals, &c. ; they are really 

 central areas for the nervous stimuli. Each nerve-cell or gan- 

 glion cell possesses a very distinct nucleus and nucleolus, and 

 one, two, or more processes, when they are known as uni-, 

 bi-, or multipolar ganglion cells. One root is always that of a 

 nerve-iilament. 



Nerve-fibres are of two kinds : one variety carries impulses 

 — sensations — from the central organ (cells) to the peripheral 

 organs, — these are called motor or secretory fibres ; the other 

 carries impulses from the periphery to the central organs, and 



