48 



THE ELEMENTS OF STRUCTURE 



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. 



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

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



The cells may be without 

 any intercellular "mortar" or 

 matrix. They may be laden 

 with fat or with pigment. 



In other cases the cells of 

 the connective tissue lie in a 

 matrix, which they secrete, or 

 into which they in part die 

 away. Sometimes the matrix 

 becomes secondarily invaded 

 by cells. The connective cells 

 are very often irregular in out- 

 line, and give off, in most cases, 

 fine processes, which traverse 

 the matrix as a network. They 

 may secrete long fibres, as in 

 the various kinds of fibrous 

 tissue. 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 Verte- 

 brate 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 

 C, Cilia; B., basal corpuscle at tlic root of (osteoblasts), which mav rest 

 •each cilium; -F-i, fi"^ intracellular hbnls cartilage foundation, or 



corresponding to the cilia ; A ., nucleus « .« ^ f, 



with chromosomes darkly stained; (Y., may be quite mdepondent. 

 general cytoplasm of the cell. 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 





Fig. 22. — Three ciliated cells. 



