THE ENDOCRINAL ORGANS 33 1 



normal, so that the patient loses weight. As too much thyroxine speeds 

 up the life processes, too little slows them down. Basal metabolism is low, 

 so that the tendency is to put on fat. In the young, both growth and 

 development are retarded; and if the deficiency is very great, a child, 

 unless given thyroid artificially, may become an idiotic dwarf not unlike a 

 cretin. 



In man, as in other vertebrates, the thyroid gland arises as a median 

 ventral outgrowth from the floor of the pharynx at the level of the first 

 visceral pouch. Eventually connexion with the tongue is lost. 



The thyroid gland is a peculiarity of chordates, and is found in all 

 classes of this phylum. In the hemichordates, the only possible homologue 

 of the thyroid is a groove, apparently functionless, in the floor of the 

 pharynx. The urochordates and cephalochordates have a ciliated groove, 

 the endostyle, in the floor if the pharynx. This groove is morphologically, 

 if not physiologically, comparable with the thyroid. 



In Amphioxus, a typical cephalochordate, the endostyle is lined by 

 columnar epithelial cells of two sorts, mucus-glandular and ciliated. 

 Particles of food caught up in the mucus are swept forward towards the 

 mouth and are carried by a ciliated circumpharyngeal groove to a median 

 epibranchial groove, which carries them posteriorly to the intestine. 



The endostyle of Petromyzon larvae, like that of Amphioxus, is a 

 mucus-secreting organ with four rows of mucus-secreting cells alternating 

 with rows of ciliated cells. This larval endostyle, however, is a transient 

 structure which develops into the thyroid gland of the adult animal. 

 Connexion of the anlage with the pharynx is eventually lost, and the organ 

 becomes vesiculated like the thyroid of higher vertebrates. The vesicles 

 secrete colloid, and the function is evidently endocrinal. The homology 

 of the thyroid gland and endostyle is further attested by the fact that, in 

 the cyclostome Bdellostoma, the median groove from which the thyroid 

 gland develops extends the entire length of the floor of the pharynx, pre- 

 cisely as does the endostyle of Amphioxus. In fishes, as in cyclostomes, 

 the thyroid is usually unpaired. A subdivision into two lobes is, however, 

 characteristic of Amphibia. 



In reptiles, the gland is again unpaired and remains unpaired in most 

 mammals, with a tendency to form lateral lobes as in man. The position 

 of the gland is fairly constant, ventral to the trachea and just below the 

 larynx. 



Evidence has been given (p. 251) that the endostyle is a modified 

 gill pouch. The history of the thyroid therefore reveals a complete change 

 of function such as we have already seen to be a general characteristic of 

 pharyngeal structures. 



Parathyroid Glands. The parathyroid glands in appearance resemble 

 lymph nodes, and there are usually four in man. They generally 



