THE THYREOID GLAND 411 



enlarged. In some cases, tumours of the thyreoid may 

 transmit the pulsations of the carotid artery. 



The internal secretions of the thyreoid gland are of the 

 greatest importance in controlling and regulating the tissue- 

 changes of the body. Congenital absence of the gland causes 

 the condition of cretinism, in which the subject is backward in 

 growth, both mentally and bodily. Myxcedema arises when 

 the secretions of the thyreoid, hitherto normal in character 

 and sufficient in amount, undergo alterations, leading to 

 changes in both the mental and the physical conditions of 

 the patient. These involve a curious overgrowth of the 

 superficial fat in certain regions of the body, notably in the 

 face and in the lower part of the posterior triangle of the neck. 

 The opposite condition, thyreoidism, is most frequently seen 

 in exophthalmic goitre. It is also met with after operations 

 which have involved incisions into the gland, and it then 

 arises from absorption of the secretion which is poured out 

 from the cut surface. The condition is characterised by a 

 rapid and weak pulse, rapid and shallow respirations, tremor 

 and other signs of great nervous excitability. It is said that 

 removal of the thyreoid gland alone does not cause myxcedema, 

 and it is suggested that the parathyreoids (p. 412), if left 

 behind, are able to carry out all the duties of the thyreoid 

 gland. 



DEVELOPMENT OF THE THYREOID GLAND. The thyreoid 

 gland arises as a downward-growing hollow bud in the floor 

 of the primitive pharynx, and its connexion with the mouth 

 remains as the foramen ccccum, a small blind pit situated in the 

 middle line on the dorsum of the tongue, at the junction of its 

 middle and posterior thirds. The bud grows downwards in 

 front of the larynx, loses its lumen and enlarges to form the 

 isthmus and the lateral lobes. The connexion between the 

 foramen caecum of the tongue and the isthmus of the thyreoid 

 is known as the thyreo-glossal duct, and it may persist either 

 in a part or in the whole of its extent. When persistent, it 

 may give rise to a thyreo-glossal cyst, which may extend 



