CHAPTER XII 

 THE PITUITARY BODY (continued) 



THE ACTIVE PRINCIPLE OR PRINCIPLES OF THE PITUITARY ' 



VARIOUS attempts have been made to isolate the active principles of this 

 organ, and more than one investigator has succeeded in obtaining from 

 extracts of the posterior lobe a crystallisable material which produces the 

 stimulation of plain muscle characteristic of such extracts. Clear, protein - 

 free, sterilised solutions containing the active principles are met with in 

 commerce under the names pituitrin, hypophysin, etc., but it is not claimed 

 that these represent pure chemical substances. There is reason to believe 

 that, unlike the suprarenal medulla, the posterior lobe of the pituitary 

 body yields more than one active autacoid (see p. 96). On the other 

 hand, no active principle has been extracted from the pars anterior. This, 

 however, does not negative the idea to which we are led by the evidence of 

 many experimental and clinical observations, that the pars anterior forms 

 a substance or substances which pass into the blood and exercise important 

 effects on metabolic processes in various organs. 



EFFECTS OF EXTRACTS OF THE POSTERIOR LOBE 



Effects on Plain Muscle. The announcement in 1895 that extracts of 

 pituitary, as well as those of suprarenal, have a remarkable influence upon 

 the vascular system, producing a great rise of blood-pressure with contrac- 

 tion of vessels and increase in force of the heart-beats (fig. 56), led to the 

 attention of physiologists being drawn to this gland, which had until then 

 been neglected by them. Howell (1898) found that the action is confined to 

 extracts of the posterior lobe which, as we now know, includes pars inter- 

 media and pars nervosa and it has since been shown that it is yielded by 

 extracts of either pars intermedia or pars nervosa alone, but rather more 

 distinctly by the pars nervosa than by the pars intermedia (Herring). The 

 effect of the extract upon the blood-vessels is a direct one, and is not 

 due, as in the case of the suprarenal hormone, to its stimulant action on 

 sympathetic endings. The effect on the heart was also shown by Howell 

 to be different from that of adrenalin ; for whereas with the vagi cut 

 or paralysed adrenalin causes a marked acceleration of the heart (sym- 

 pathetic stimulation), the pituitary autacoid causes slowing with increase 

 in force of the individual beats. In the bird it augments both auricular 



o 



and ventricular contractions, whereas adrenalin affects the auricles only 



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