THE CONNECTIVE TISSUES. 



97 



physical properties. Bone and cartilage serve as supporting tissues ; 

 the looser fibrous tissues for binding and holding the organs and 

 parts of organs firmly in place. The denser fibrous connective tis- 

 sues come into play where strength and pliability are desired, as in 

 ligaments, or else are used in the transmission of muscular force, as 

 in tendons. Another important characteristic of connective tissue is 

 that its various members are capable of undergoing transformation 

 into wholly different types ; bone, for instance, being developed from 

 fibrous connective tissue and from cartilage. Certain structures are 

 represented by different members of the connective-tissue group in 

 the different classes of vertebrates. In certain fishes the skeleton is 

 cartilaginous, and in certain birds the leg tendons are formed of 

 osseous tissue, etc. The connective tissues receive their nutrition 

 from the lymph. In the 

 denser connective tissues this 

 permeates the tissues through 

 clefts or spaces in the ground- 

 substance, in which the con- 

 nective-tissue cells are found 

 and which are united by 

 means of fine canals into a 

 canalicular system. In the 

 looser fibrous tissues and in 

 mucous connective tissue the 

 system of lymph-channels is 

 not present; here the lymph 

 seems to pass through the 

 ground-substance. 



. Certain connective-tissue 

 cells have the function of 

 producing fat. In various 

 parts of the body, masses of 

 fat tissue are formed as a 



Fig. 60. Mesenchymatous tissue from the 

 subcutis of a duck embryo ; X ^5- 



protection to various organs 



and as a reserve material upon which the body can call when ne- 

 cessary. This type can hardly be considered a separate class of 

 connective tissues, as it can be demonstrated that it is merely modi- 

 fied connective tissue, and can occur wherever the latter is found. 

 Finally, certain elements of the middle germinal layer are capable 

 of producing colored substances known as pigments. To this class 

 belong the pigment cells and the red blood-corpuscles. 



All the members of this group are developed from the mesen- 

 chyme, an embryonic tissue developed early in embryonic life from 

 the middle germ layer or mesoderm. In its early development the 

 mesenchyme is probably composed of individual cells. As develop- 

 ment advances the protoplasm of these cells increases, and is united 

 by means of the protoplasmic branches formed by the cells to form 

 a protoplasmic complex known as a syncytium. The further 

 7 



