48 THE GLANDS REGULATING PERSONALITY 



Alongside are shown several examples of the amblystoma stage, 

 produced in one of the laboratories of Oxford University and at 

 the gardens by thyroid feeding. A variation of the thyroid in 

 the direction of increased secretion was probably responsible for 

 the first land animals. 



Thyroxin, Secretion of the Thyroid 



Under the microscope, as in the test tube, the thyroid shows 

 remarkable and unique features. Closed spherules lined by a 

 single layer of cells enclosing a gelatinous material known as 

 colloid, which stains deeply with acid dyes, comprise the units of 

 its architecture. Essentially, it may be pictured as a series of 

 jelly bubbles secreted by outlying cells. 



A relatively high percentage of iodine is the unique distinctive 

 fact in its chemistry. Discovered by Baumann in 1895, the 

 presence of the element has focused the intelligence of chemists 

 upon the gland, with the consequent demonstration of arsenic 

 also in it. It was soon manifest that the secretion of the gland 

 was dependent upon the iodine content for its activity. Active 

 extracts of the thyroid like thyreoglobulin and iodothyrin were 

 proven to contain iodine, and to become inactive when the iodine 

 was removed. Efforts to isolate the iodine containing active 

 principle in pure form were fruitless until the work of Kendall 

 at the Mayo Foundation. He obtained it as a white, finely 

 crystalline, odorless and tasteless substance, heat stable, and 

 analyzable. The free form separates as a sheaf of fine needles. 

 Kendall at first called it the a-iodine compound, then named it 

 thyroxin. 



There are other internal secretions of the thyroid, with a func- 

 tion ef their own, that have no iodine. But they are secondary, 

 and obscure. Thyroxin is accepted today as the purified internal 

 secretion of the thyroid because all the effects of the whole gland 

 may be elicited with it. Thyroxin produces results with doses 

 amazingly minute compared with the quantity of whole gland 

 necessary. Moreover, a dose of thyroxin appears to last an 

 organism in need of it over a period of time; the other has to be 

 administered continuously. 



Studies with thyroxin carried on in recent years have rounded 

 out the whole concept of the business of the thyroid in the body 

 my. One may sum it up by saying that the thyroid secre- 

 tion is the great controller of the speed of living. The more thy- 



