42 THE ELEMENTS OF STRUCTURE. 



ol 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 concentric 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 back - 

 boneless animals. 



(c) Muscular tissue. The single-celled Amceba 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 Ccelentera 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 Ccelentera 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 

 Ccelentera. 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 * ' mesen- 

 chyme " 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 Amoeba, we recognise 

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

 of the cell to another. 



