MUSCULAR TISSUE. 35 



In Hydra and some other Coelenterates, the bases of the epithelial 

 cells which form the outer layer and line the internal cavity, are pro- 

 longed into contractile roots. By the activity of these muscular roots 

 the Hydra elongates and contracts its body. Here then we have cells 

 of which a special part discharges a contractile or muscular function, 

 while the other parts__of the cells retain other powers. (The external 

 contractile cells in Hydra are often called " neuro-muscular," as if they 

 combined nervous and muscular functions. It is possible that they do, 

 but the existence of special nerve-cells dogs not favour the idea in this 

 case at least. ) 



In other Coelenterates, the muscular cells are still directly connected 

 with the epithelium, but become more and more exclusively devoted to 

 the function of contraction. In iill. other animals the muscular tissue 

 is derived from the mesoderm, which, as we have already mentioned, is 

 not distinctly present in Coe lenter atgs. In the majority the muscle-cells 

 arise on the walls of the body-cavity, and their origin may often at least 

 be just^ described as epithelial. But in other cases the muscles are 

 startedriy those wandering " mesenchjmae " cells, to which we have 

 already referred. -^5=— 



So, ps regards origin, muscular tissue may be classified as follows : — 



1. In Coelenterates, where there is no definite "mesoderm," the 

 muscle-elements arise from the ectodermic and endodermic 

 epithelium, of which they often form a direct part. In the 

 passive Sponges, the contractile elements are few and un- 



important. 



2. In other animals the muscle-elements arise from the middle 



layer, but O. and R. Hertwig distinguish between those which 

 have an "epithelial" and those which have a " mesenchy- 

 matous " origin. 



Structure. — A distinction is usually drawn between striped and un 

 striped muscle-fibres, but, according to the Hertwigs, this distinction is 

 of little morphological value. 

 USmooth or unstriped muscle-fibres are elongated contractile cells, ex- 

 ternally homogeneous in appearance. They are especially abundant in 

 sluggish animali, e.g.. Molluscs, and occur in the walls of the gut, 

 bladder, and blood-vessels of Vertebrates, where they are somewhat 

 quaintly called "involuntary."' They are less perfectly differentiated 

 than striped muscle-fibres, and usually contract more slowly. 



A striped muscle-fibre is a cell, the greater part of which is modified 

 into a set of parallel longitudinal fibrils, with alternating " clear and 

 dark " transverse stripes. A residue of unmodified cell-substance, with a 

 nucleus or with many, i? often to be observed on the side of the fibre, 

 and a slight sheath or sarcolemma forms the ' ' cell-wall. " Many muscle- 

 fibres closely combined, and wrapped in a sheath of connective tissue, 

 form a muscle, which, as every one knows, can contract with extreme 

 rapidity when stimulated by a nervous impulse. 



The contraction involves a visible change of form, associated with a 

 chemical " explosion " in the cell-substance with the production of heat 

 and waste-products, and with subtle changes of electric potential. One 

 of the greatest marvels of animal life, is the strength and sustained 

 power of muscles. 



