MUSCULAR TISSUE 



49 



the blood, and those of the body cavity or " perivisceral " fluid 

 which is often abundant and important in backboneless animals. 



(c) Muscular tissue. — The single-celled Amoeba moves by flowing out 

 on one side and drawing in its substance on another. It is diffusely 

 contractile, and it has also sensitive, digestive, and other functions. 



In Hydra and some other Coelentera the bases of some of the epithelial 

 cells which form the outer and inner layers 

 are prolonged into contractile roots. Here, 

 then, we have cells of which a special part 

 discharges a contractile or muscular func- 

 tion, while the other parts retain other 

 powers. 



In other Coelentera the muscular cells are 

 still directly connected with the epithelium, 

 but become more and more exclusively con- 

 tractile. In all other animals the muscular 

 tissue is derived from the mesoderm, which, 

 as we have already mentioned, is not dis- 

 tinctly present in Coelentera. In the 

 majority, the muscle-cells arise on the walls 

 of the body cavity, and their origin may 

 often at least be described as epithelial. 

 But in other cases the muscles arise from 

 those wandering " mesenchyme " cells to 

 which we have already referred. 



Smooth or unstriped muscle fibres are 

 elongated contractile cells, externally homo- 

 geneous in appearance. They are especially 

 abundant in sluggish animals, e.g. Molluscs, 

 and occur in the walls of the gut, bladder, and 

 blood vessels of Vertebrates. 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, is 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. 



(d) Nervous tissue. — Beginning again with the Amoeba, we recognise 

 that it is diffusely sensitive, and that a stimulus can pass from one part 

 of the cell to another. 



In some Coelentera a few of the external cells seem to combine 

 contractile and nervous functions. Therefore they are sometimes called 

 " neuro-muscular." 



Fig. 23. — A 

 unstriped 



smooth or 

 muscle-cell. 



slowly contracting. 



A'^., Nucleus ; FL., longitudinal 

 intracellular fibrillation. 



