34 THE ELEMENTS OF STRUCTURE. 



(b) Connective Tissue. 



I am afraid that this term is somewhat like the title "worms." It 

 includes too many different kinds of things to mean much. It represents 

 a sort of histological lumber-room. 



The embryologists help us a little, for they have shown that 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 embryo (ectoderm or endoderm), 

 we may say that connective tissue is primarily derived from epithelium. 

 Sometimes, e.g., in the lancelet [Amphioxus], this origin is very evident. 

 Moreover, in the Sponges and Stinging-animals which have no strict 

 mesoderm, there is usually a middle stratum ("mesogloea"), of a. 

 gelatinous character in jellyfishes, with more abundant cells in Sponges. 

 It must be included as a form of " connective tissue," and it is derived 

 from the outer and inner layeis which enclose it. 



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 cjpse together, without any intercellular 

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

 duce the appearance of a network, or they may be crammed 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, are good illustrations of 

 connective tissue with much matrix. Cartilage is sometimes hardened 

 by the deposition of lime salts in its substance, and then has a slight re- 

 semblance 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. 



Origin.— Tas. single-celled Ammba 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. 



