628 SPECIAL PHYSIOLOGY. 



Intrinsic absorption is sometimes simply interstitial, accomplishing 

 the removal of tissues, molecule by molecule, without any solution of 

 continuity, or breach of substance, in them, the part affected becoming 

 merely smaller, and not necessarily undergoing any special change of 

 form. During simple interstitial absorption, nutritive changes, involv- 

 ing the deposition of fresh material, must still go on, but the process 

 of absorption is relatively more active than that of the deposition of 

 new matter. This kind of interstitial absorption is illustrated in the 

 wasting which takes place as the result of hunger or starvation, and 

 also in the disease known as atrophy. 



Another form of intrinsic absorption, known as progressive absorp- 

 tion, involves more or less solution of continuity, or breach of substance. 

 It is often apparently caused by pressure interfering with the nutrition 

 of a part ; it is exemplified in certain morbid processes, as when an 

 aneurismal, or other deep-seated tumor, in approaching the surface, 

 induces absorption of the interposed structures, even the bones being 

 absorbed under the effects of constant pressure. Abscesses also tend 

 to the surface of the body or of internal mucous cavities, by a similar 

 progressive absorption. Another form of this process is named dis- 

 junctive absorption ; in this, the living part of a tissue, in immediate 

 connection with a dead portion, is removed by absorption, and so the 

 dead part is detached ; such a process occurs in the separation of a 

 slough from a soft tissue, or of a necrosed or dead portion of a bone, 

 from a living part, and also in the throwing off of a portion of the 

 entire limb, as in the case of gangrene of the foot. 



Certain tissues also undergo intrinsic absorption much more readily 

 than others. Bone, one of the hardest tissues in the body, is very 

 readily absorbed ; its numerous Haversian canals, and cancelli, and 

 even its general medullary cavities, are channels or spaces produced, 

 during its growth, by an absorptive excavation of a previously solid 

 osseous tissue ; such changes occur in it, even when it is fully developed ; 

 and, as just now stated, it is very easily absorbed under abnormal 

 pressure. The fangs of the temporary or milk teeth, which are com- 

 posed of dentine, a substance more compact than bone itself, undergo 

 progressive absorption under the influence of pressure from the summits 

 of the rising permanent teeth, and, in this way, are loosened, and finally 

 drop away from the gum. Cartilage is less easily absorbed than bone, 

 but, nevertheless, it does yield to that process. The fasciae, areolar 

 {tissue, skin, and mucous membranes, also give way under the progres- 

 sive absorption caused by abscesses which are advancing to the sur- 

 face ; the epidermis and epithelium, however, burst mechanically. 

 Vascularity is necessary for the occurrence of true intrinsic or pro- 

 gressive absorption. Cartilage is probably absorbed by closely adja- 

 cent vessels. All vascular organs and tissues are liable to progressive 

 absorption under pressure, and all may undergo waste or atrophy. 



In the progress of development, in Man and animals, many instances 

 occur of the disappearance of parts not permanently needed, such as 

 the temporary gills of the higher Amphibia, the tails of the tadpoles 

 of the anourous species, and also certain large bloodvessels which are 

 mo longer required in more advanced conditions of development. The 



