Chap, iv.] METABOLIC PROCESSES OF THE BODY. 601 



difficulty in ensuring the removal of the whole gland; but this 

 is not wholly clear. 



The view that the thyroid effects some change on the blood 

 passing through it is further confirmed by clinical experience. 

 The disease known as myxoedema, characterized by disordered 

 nutrition, notably of the skin and of the nervous system, but 

 also of other parts of the body, is closely associated with morbid 

 changes of the thyroid body, and thus is allied to goitre and 

 cretinism. The symptoms in many respects resemble those 

 produced in animals by removal of the thyroid. Now in such 

 cases the symptoms are in a most remarkable way lessened or 

 even removed by the systematic subcutaneous injection of the 

 extract of the fresh thyroid body of an animal, or even by 

 the extract being taken regularly by the mouth. The small 

 quantity of substance thus introduced into the blood is sufficient 

 to modify the altered nutrition of the body, and to bring it back 

 to its normal condition. The inference is that under normal 

 conditions the thyroid gives up in some way to the blood the 

 substance or substances which in the above instance are arti- 

 ficially administered in the thyroid extract, and the presence of 

 which is in some way essential to the normal nutrition of the 

 body. 



What that substance is or those substances are, and how they 

 act, we are not yet in a position to say. The characteristic 

 presence in the alveoli of the thyroid of mucin, or of a substance, 

 the so-called " colloid " having at least a superficial resemblance 

 to mucin, and the fact that in myxoedema a mucin-like body is 

 in excess in the tissues, hence the name, have led to speculations 

 as to the connection of the thyroid and mucin. But our knowl- 

 edge is not at present such as to justify any definite statement. 



§ 393. The Pituitary Body. The lower, posterior, lobe of 

 this organ in many respects resembles the thyroid body (the 

 upper, anterior, lobe is of quite distinct nature, being really 

 a part of the central nervous system), but concerning the proc- 

 esses which take place in this lobe and the purposes of the 

 organ as a whole we know absolutely nothing. 



§ 394. The Suprarenal Bodies. These differ wholly in 

 structure from the thyroid body. The two parts of which 

 the body consists, cortex and medulla, are not, like the cortex 

 and medulla of a lymphatic gland, different arrangements of 

 the same material, but are of essentially different nature and 

 indeed are of different origin. The medulla is derived from, 

 is a modification of, sympathetic ganglia, while the cortex is 

 derived from masses of mesoblastic cells surrounding the great 

 blood vessels ; and in some animals the two form wholly separate 

 bodies. 



Some of the histological features of the suprarenal bodies, 

 namely the groups of cells and their abundant blood supply, 



