SELF-REGULATING MECHANISM. ' 327 



impressions to the nerve centres from which the 

 efferent or motor branches distributed to the coats of 

 the arteries spring. In many tissues and organs in 

 which the circulation is easily disturbed by peripheral 

 irritation the nerve fibres distributed to capillary 

 vessels are exceedingly numerous. 



311. Of the self-acting mechanism by which the 

 supply of blood to tissues is regulated. We are now 

 in a position to consider the probable arrangement 

 of the reflex nerve mechanism by which the flow of 

 blood in the capillaries is regulated, and the equable 

 distribution of nutrient fluid to the tissues outside the 

 capillaries is ensured in a state of health. We shall 

 also see how this elaborate mechanism is rendered 

 self-acting, self-regulating, and self-adjusting during 

 the healthy state. 



As is well known, the nutritive operations of man 

 and most vertebrata have to be carried on with com- 

 paratively little alteration under very variable and 

 continually changing external conditions. A limited 

 range of variation is permitted, but if the limits 

 within which this self-adjusting apparatus acts per- 

 fectly, be overstepped in either direction, as not 

 unfrequently happens under the very variable and arti- 

 ficial conditions to which civilised man and domestic 

 animals are exposed, the range of the capacity for 

 self-adjustment is exceeded, the mechanism is strained, 

 and a part temporarily thrown out of order, or seriously 

 damaged. Unfortunately repair can only be imper- 

 fectly carried out, and the arrangement restored to its 

 normal state in cases in which the injury is compara- 

 tively slight. So complex is the mechanism and so 

 widely distributed are its interdependent parts that 

 renovation after its destruction is complete is im- 

 possible, and I doubt of its re- formation in the adult 

 has ever occurred, or is indeed possible. 



In the diagram in Plate XXI, I have collected and 

 arranged the facts ascertained by anatomical investi- 



