22 



NORMAL HISTOLOGY. 



During the period of embryonal growth the intercellular substance is 

 semifluid, gelatinous and plastic; a little later, as growing connective tissue, 

 it is still soft, although more definitely formed; while, as the adult areolar 

 tissue, it becomes tough and yielding. Grouped as masses in which fibrous 

 tissue predominates, the intercellular substance acquires the toughness and 

 inextensibility of tendon; where, on the contrary, large quantities of elastic 

 tissue are present, as in certain ligaments, extensibility is conspicuous. 

 Further condensation of the intercellular substance produces the resistance 

 encountered in hyaline cartilage, intermediate degrees of condensation being 

 presented by the fibrous and elastic varieties. In those cases in which the 

 ground-substance becomes impregnated with calcareous salts, the hardness 

 of bone or of dentine results. Notwithstanding these variations in the 

 density and physical properties of the intercellular substance, the cellular 



elements have undergone little 

 radical change, the connective 

 tissue-corpuscle, the tendon-cell, 

 the cartilage-cell and the bone- 

 corpuscle being essentially iden- 

 tical. 



The principal forms in which 

 connective tissue occurs are: (i) 

 Mucous Tissue, (2) Reticular Tis- 

 sue, (3) Fibrous Tissue loose 

 and dense, (4) Adipose Tissue, 

 (5) Cartilage and (6) Bone. 



Mucous Tissue. This 

 form of connective substance is 

 the most immature, being in fact 

 the embryonal type, and closely 

 resembles the parent tissue, the 

 mesenchyma. As seen in sec- 

 tions of the embryo or of the early umbilical cord, it consists of a delicate 

 protoplasmic network containing a semifluid intercellular substance. The 

 network is formed by the union of the processes of irregularly branched 

 stellate or fusiform cells, whose oval nuclei are embedded in plate-like 

 masses of faintly granular cytoplasm. The intercellular ground-substance 

 is semifluid and, depending upon the stage of development, either struct- 

 ureless or traversed by indistinct fibrillae. The latter owe their origin to 

 the cells and are produced by differentiation of the cytoplasm. Being 

 essentially embryonal tissue, in the higher animals the mucous variety 

 of connective tissue is limited to the earlier stages of development, the 

 so-called jelly of Wharton in the young embryo being a striking example. 

 Among the invertebrates, on the other hand, mucous tissue is formed in 

 the adult animal. Certain pathological growths, known as myxoma, exhibit 

 a similar arrangement of cells and yield mucus. The latter substance, 

 produced also by glandular epithelium, contains true mucins a group of 

 complex proteid substances. 



Reticular Tissue. This variety of connective tissue differs from the 

 mucous in retaining only a very meagre amount of intercellular substance 

 and consists, therefore, chiefly in some instances almost exclusively of a 

 network of* connective tissue cells, the meshes of which are occupied by fluid 

 and the lymphoid elements which the reticulum supports. The cells are 

 flat and stellate and rest upon the surface of the strands of intercellular sub- 



FiG. 27. Mucous tissue from section of very young 

 umbilical cord. X 350. 



