33 2 THE DUCTLESS GLANDS. [CH. xxm. 



which was performed previous to our knowledge of the importance 

 of the thyroid, is not regarded as justifiable nowadays. 



Lastly, in many animals removal of the thyroid produces 

 analogous symptoms, in the overgrowth of the connective-tissues 

 especially under the skin, and in the nervous symptoms (twitch- 

 ings, convulsions, etc.). 



The term Myxcedema was originally given under the erroneous 

 idea that the swelling of the body is due to mucin. In the early 

 stages of the disease there is a slight increase of mucin, because 

 all new connective-tissues contain a relatively large amount of 

 ground substance, the most abundant constituent of which, next 

 to water, is mucin. But there is nothing characteristic about 

 that. 



The discovery of the relationships between the thyroid and 

 these morbid conditions is especially interesting, because impor- 

 tant practical results in their ti-eatment have followed close on 

 the heels of experimental investigation. The missing internal 

 secretion of the thyroid may be replaced in these animals and 

 patients by grafting the thyroid of another animal into the 

 abdomen ; or more simply by injecting thyroid extract subcu- 

 taneously ; or even by feeding on the thyroid of other animals. 

 This treatment, which has to be kept up for the rest of the 

 patient's life, is entirely successful. Chemical physiologists have 

 been searching recently to try and discover what the active 

 material in thyroid extract is which produces such marvellous 

 results ; the view at present held is that the efficacy of thyroid 

 extract is due to a substance which Baumann separated from the 

 gland and which stands almost unique among physiological com- 

 pounds by containing a large percentage of iodine in its molecule. 

 Thyro-iodin or lodo-thyrin, as this substance has been called, is 

 present in combination with proteid matter in the colloid 

 substance. 



Intravenous injection of thyroid extract in a normal animal 

 lowers blood pressure ; in an animal from which the thyroid 

 has been removed injection stimulates the heart and raises blood 

 pressure. 



Parathyroids. These are small bodies situated in the neigh- 

 bourhood of, or embedded in the substance of, the thyroid. They 

 are made up of elongated groups of polyhedral cells bound 

 together by connective-tissue and well supplied with blood- 

 vessels. Some observers look upon these as being even more 

 essential to healthy life than the thyroid, but this point is by no 

 means decided. 



The general idea, however, that pi-evails is that the thyroid 



