34 DEVELOPMENT AND FUNCTION OF MACROPHAGES IN BONE-REPAIR. 



move toward it, or which brings about the dislodgment of a lymphocyte from its 

 ordinary depot into the blood-stream (for we can hardly regard the blood-stream 

 as normally containing sufficient numbers of these cells to supply the requirements 

 without replenishment)? The specific stimulant is undoubtedly the product of 

 tissue breakdown. It may operate through its chemical properties, in which case 

 its action would be described as a chemiotaxis, or its effect may be due to its pecu- 

 liar physical condition and its influence be more accurately designated as a physi- 

 cotaxis. Be that as it may, it is certain that the cells themselves are in some way 

 specialized to react to this form of stimulation, not only by moving toward the 

 source of the attraction in the soft parts of the vicinity by amoeboid movement 

 through the tissues, in the blood by movement of lymphocytes from their resting- 

 places into the blood-stream and from this into the inflammatory area but by 

 developing prodigious phagocytic abilities. 



In connection with the hematogenous lymphocytes the attraction must be 

 thought of as acting to get the cells from their depots into the blood-stream, where 

 the attracting influence may be assumed to be circulating. But the latter would not 

 control their course in the blood-current, for in this they float passively until they 

 happen to reach the region of inflammation. Here the circulatory conditions favor 

 their arrest, the blood-current in the dilated capillaries being very slow. Diapedesis 

 ensues, and the embryo phagocytes are thus assembled on the field of their opera- 

 tions. The inflammation-area, as it were, "screens out" the lymphocytes from the 

 blood, just as, in certain types of septic inflammation, the leucocytes which are 

 brought in the blood to the focus of inflammation are sifted out and retained there 

 by the mechanical and other conditions which they encounter. 



The interesting fact is noted by Tschaschin and others that the local lym- 

 phoid elements of adenoid-tissue do not normally stain with vital dyes and even 

 in inflammation they are very slow in developing phagocytic power. It appears, 

 however, that if these cells gain entrance into the blood-stream they soon become 

 sensitive and react promptly to the stimulus of inflammation by wandering from 

 the vessels and becoming metamorphosed into typical "polyblasts." 



ACTIVITY. 



The method of action of the macrophages has long been the subject of much 

 study on the part of cytologists. There is nothing to suggest that these phagocytes 

 actually break down the tissue. It may be that they secrete some enzyme which 

 assists in this process, but no facts were discovered in support of this view. Their 

 function is concerned with the clearing away of the waste products rather than with 

 tissue solution. In the section describing the observations it has been noted that 

 the cells are not typically in direct contact with the tissue being absorbed. More- 

 over, they do not seem to operate merely by engulfing fragments of the moribund 

 tissue, although their ability to act in this way on occasion is not questioned ; rather, 

 they are concerned with the imbibition of a colloidal solution of the tissue. In this 

 solution they lie, their outer walls bathed with it. The way in which they absorb 

 the material may be inferred from the way in which they are known to take up col- 



