Integrative Systems 



6 



It is necessary for an organism that its various organs should work 

 together harmoniously. The more complex an organism, the more 

 complicated must be the mechanism which effects functional corre- 

 lation among its different but interdependent parts. This functional 

 integration of the highly complex vertebrate body is secured by two 

 quite different mechanisms, one nervous, the other glandular. The 

 structure of the nervous system is such as to suggest some analogy 

 with telephonic or telegraphic systems of communication. By means 

 of nerves, each a bundle of exceedingly delicate nerve fibers, "mes- 

 sages" — i.e., nervous impulses — are transmitted from one part of the 

 body to another. The glandular mechanism operates by transportation 

 of chemically specific secreted substances via the blood-stream. The 

 glands of this system are called "ductless glands" because the specific 

 products of its secreting cells diffuse directly into the blood which is 

 flowing through the capillaries of the gland. The secreted substance 

 acts as a "chemical messenger" (hormone), inducing characteristic 

 and appropriate reactions in various structures to which it is carried 

 by the blood. Nervous transmission is almost instantaneous. The 

 speed of the blood-current is relatively slow. Therefore, in general, 

 quick adjustments, correlation of muscular activities, and responses 

 to stimuli received from the external environment are effected by the 

 nervous system. But the relatively slow adjustments, and such 

 processes as metabolism and growth, are largely controlled by the 

 ductless or endocrinal glands. It has recently been discovered that 

 the stimulating action of a nerve, at least in some cases, depends on 

 release, by the nerve-endings, of minute quantities of chemically 

 specific substances which are of the nature of hormones and act in 

 the manner of hormones. The existence of these "neurohumors" makes 

 it appear that the difference between nervous action and endocrinal 

 action may be mainly structural. 



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