38 THE ELEMENTS OF STRUCTURE. 



(effecting locomotion, food-wafting, &c.), or sensitive (and as sucli 

 forming sense organs), or glandular (liberating certain products or even 

 the whole contents of its cells), or pigmented (and thus associated W'lth 

 respiration, excretion, and protection), or covered externally with a 

 sweated-off cuticle, susceptible of many modifications (especially of 

 protective value). 



(b) Connective Tissue. 



This term is somewhat like the title " worms." It includes too many 

 different kinds of things to mean much. It represents a sort of histo- 

 logical lumber room. 



The embryologists help us a little, for they have shown that almost 

 all forms of connective tissue are derived from the mesoderm or middle 

 layer of the embryo. As this mesoderm usually arises in the form of 

 outgrowths from the gut, or from ("mesenchyme") cells liberated at 

 an early stage from either (?) of the two other layers of the embrj'O 

 (ectoderm or endoderm), we may say that connective tissue is primarily 

 derived from epithelium. 



The general function of " connective tissue " is to enswathe, to bind, 

 and to support, but the forms assumed are very various. 



(a) The cells may be close together, without any intercellular 

 " mortar " or matrix. They may contain large vacuoles, and thus 

 produce the appearance of a network, or they may be laden with fat 

 or with pigment. 



{b) In other cases the cells of the connective tissue lie in a matrix, 

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

 very often irregular in outline, and give off 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 " — Ijone. 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 veiy 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 " peri\isceral " fluid, which is 

 often abundant and important in backboneless animals. 



(c) Aliiscular Tissue. 



Origin. — The single celletl Aiiurba 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 Cojlentera the bases of some of the epithelial 



