NORMAL HISTOLOGY. 



FIG. 30. Lymph-spaces within dense connective tissue, 

 from cornea of calf ; the surrounding ground-substance has 

 been stained with argentic nitrate. X 500. 



(Fig. 30), in which are lodged the connective tissue cells, applied to the 

 wall formed by the dense fibrous tissue. In principle, the same arrangement 

 holds good for cartilage and bone, since in these tissues the cells lie within 

 the lacunae. The larger branched connective tissue cells sometimes, as when 



subjected to thermal, chemical 

 or electrical stimulus, exhibit 

 changes in form, possessing 

 the power of retracting their 

 processes and displaying feeble 

 amoeboid movement. 



In a few localities in man 

 the choroid, the iris, the sclera, 

 the dermis and the pia mater 

 but widely distributed in the 

 lower vertebrates, the branched 

 connective tissue elements often 

 contain dark particles of mela- 

 nin and, therefore, appear as 

 conspicuous irregular figures 

 shading from brown to black. 

 Such elements are usually 

 spoken of as pigment-cells, be- 

 ing, of course, only connective 

 tissue cells modified by the 

 invasion of the colored foreign material. Since this invasion is limited to 

 the cytoplasm, the unaffected nucleus appears as a small light oval area in 

 the midst of the dark figure (Fig. 32). In the amphibians a favorite situa- 

 tion of pigmented cells is the immediate vicinity of blood-vessels, and it is 

 probable that at times the connective tissue cells, as well as leucocytes, may 

 take up colored particles derived from the blood. In addition to the melanin 

 series and the hemoglobin derivatives, a third group of pigments, the lipo- 

 ckromes, is derived from fat. A very common modification of the connective 

 tissue element is the appearance of 

 droplets of oil within its cytoplasm. 

 When such invasion becomes ex- 

 tensive, the element becomes a 

 fat-cell and a constituent of adi- 

 pose tissue. Further consideration 

 of the fat-cells will be deferred 

 until adipose tissue is described 

 (page 29). 



In addition to the character- 

 istic connective tissue cells and 

 their modifications containing pig- 

 ment and fat, a variable number of 

 free cells are encountered in the 

 less dense forms of fibrous tissue. 

 Much uncertainty exists as to the 

 nature and source of some of these 

 elements, and consequently it is impossible to state definitely their relations. 

 The most constant of these free cells are the migratory lymphocytes, which 

 escape from the blood-vessels into the interfascicular clefts. Being unat- 

 tached to the fibres, they change their position within the tissue and, hence, 



FIG. 31. Connective tissue cells, from cornea of calf, 

 which occupy spaces similar to those shown in preced- 

 ing figure. X 525- 



