PROGRESSIVK TlSSm-; IIIANCI-.S. 181 



by the osteoclasts, and produce fibrous lauiellae which are later 

 calcified. This process continues until the canals or cavities are 

 filled with lamellae excepting a small central cavity which con- 

 tains blood vessels, thus Haversian systems are fre(|uently com- 

 pletely regenerated. Excess of osseous tissue formed over and 

 around bones at the line of fracture (provisional callous), is 

 usually later reabsorbed. 



Adipose tissue is not a ty]iical ])riniarv tissue. It is derived 

 from the undifterentiated coimective tissue cells hv the conver- 

 sion of their protoplasm into fat. Adipose tissue is consumed 

 when the food supply is deficient, and the cells become topical 

 connecti\'e tissue cells or are destroyed. Adi])ose tissue is also 

 formed when the food supply exceeds the demand as a result of 

 production and accumidation of fat in the connective tissue cells. 



Dentine is not replaced except in some of the lower animals. 



Epithelium of surfaces is constantl_y destroyed and regen- 

 erated. The outgrowth and shedding of the superficial epi- 

 dermal cells is a physiologic process. Kpithelizati(jn of small 

 abrasions of the epidermis and mucdus membranes is rajiid and 

 complete, the regenerating cells having their origin from the 

 epithelium bordering the injur}'. If the denuded surface is large 

 regeneration may proceed from the cells of the sweat glands 

 of the skin, or mucous glands of mucous memljranes as well as 

 the epithelium Ijordering the injury. 



Squamous epithelium is more completely regenerate<l than 

 columnar. Constant destruction of columnar cells may cause 

 the production of short colunmar cells and finally squamous 

 cells. This, however, is rare, as the law of specificity is practi- 

 cally wdthout exception. Glandular epithelium is not regenerated 

 as perfectly as surface epithelium (excepting the sweat glands, 

 oil glands, mucous glands, gastric glands, Brunner's glands, 

 crypts of Lieberkiihn and uterine glands). Any of the foregoing 

 will regenerate from small islands of cells either in the duct or 

 body of the gland. The epithelium of the mammary gland in- 

 creases in amount during lactation and diminishes when lacta- 

 tion ceases. By obser\-ation it has been determined that mam- 

 mary epithelium regenerates after it has Ijeen destroyed by ab- 

 scess formation or other destructive processes, provided newly 

 formed fibrous tissue is not substituted. By analogy it might 

 be supposed that the destroyed epithelium of salivary glands 

 and of the pancreas may be regenerated, but this has not been 

 clinically or experimentally demonstrated. The liver is a tubu- 

 lar gland and regeneration of a single cell or a few cells is not 



c 



