42 THE ELEMENTS OF STRUCTURE. 



which they exude, or into which they in part die away. Such cells are- 

 very often irregular in outline, and give oft", in most cases, fine processes, 

 which traverse the matrix as a network. The fibrous tissue of tendons 

 and the different kinds of gristle or cartilage illustrate connective tissue 

 with much matrix. Cartilage is sometimes hardened by the deposition 

 of lime salts in its substance, and then has a slight resemblance to 

 another kind of "connective tissue " bone. But bone, which is 

 restricted to Vertebrate animals, is quite different from the cartilage 

 which it often succeeds and replaces. It is made by strands or layers 

 of special bone-forming cells (osteoblasts), which may rest on a cartilage 

 foundation, or may be quite independent. These osteoblasts form the 

 bone matrix, and some of them are involved in it, and become the 

 permanent bone cells. These have numerous radiating branches, and 

 are arranged in layers, usually around a cavity or a blood vessel. (There 

 are no blood vessels in cartilage.) The matrix becomes very rich in 

 lime salts (especially phosphate) ; and the cartilage foundation, if there 

 was one, is quite destroyed by the new formation. Here we may also 

 note two important fluid tissues, the floating corpuscles or cells of 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 con- 

 tractile roots. Here, then, we have cells of which a special part 

 discharges a contractile or muscular function, 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 contractile. In 

 all other animals the muscular tissue is derived from the mesoderm, 

 which, as we have already mentioned, is not distinctly 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 homogeneous 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 Ainccba, we recognise 



