PHYSIOLOGICAL 293 



ever, that the various hormones differ widely from one another 

 in chemical composition. 



(3) A negative character which they all exhibit is that they do 

 not evoke the formation of anti-bodies or counteractives in the 

 blood. That would indeed be a contradiction in terms ! » 



(4) Hormones are mainly known by the consequences that follow 

 their artificial introduction into the body, either by the injection 

 of extracts or, in some cases, by their introduction along with the 

 food. Another method of discriminating the function of a hormone 

 is to observe the changes that follow the removal or degeneration 

 of the ductless gland that produces the substance in question. 

 Both methods require careful handling, for they involve serious 

 disturbance of physiological balance. Even if an injection of pituitary 

 body always produces a particular effect on the body, it does not 

 necessarily follow that this indicates the normal use of the pituitary 

 hormone in the everyday normal life of the organism. 



The problem is also complicated by the facts that one and the 

 same endocrinal organ may produce more than one hormone, and 

 that one and the same hormone may produce several effects. Some 

 hormones seem to produce opposite results in different parts of 

 the body. Thus, as Schafer points out, adrenalin causes contraction 

 of the smooth muscle-cells of the blood-vessels, but inhibition in 

 those of the intestinal wall. "The possibility of the same autacoid 

 substance acting under some circumstances as a hormone or excitant, 

 and under other circumstances as a chalone or depressant, must be 

 borne in mind. This, indeed, serves to illustrate the drug-like nature 

 of these principles, for such inversion under different circumstances 

 is known to occur with some alkaloids." From what we have said 

 the student will infer that it is necessary to be very careful in 

 making general statements in regard to hormones. 



THE THYROID GLAND. — This ancient structure, found 

 throughout Vertebrates, is represented in man and mammals by 

 a small reddish paired body lying beside the larynx. It has a very 

 rich vascular supply. In birds it is situated in front of the heart, 

 close to the origin of the carotid arteries. In lizards it is usually 

 a paired body half-way up the windpipe; in Bony Fishes or Teleosts 

 it is paired and situated near the first gill- arch; in the skate it is 

 unpaired and situated immediately behind the lower jaw. In short,, 

 it occupies diverse positions in the region of the neck in different 

 types of Vertebrates. 



Its development is very interesting, for it always arises as a 

 mid-ventral pouch from the wall of the pharynx. This outgrowth 

 becomes constricted off from its origin, though the stalk connecting 

 it with the pharynx or the posterior floor of the mouth may persist 

 for a considerable time. It usually becomes a paired body; it becomes 



