458 COMPARATIVE ANATOMY 



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, precisely 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. 335) 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 character- 

 istic of pharyngeal structures. 



THE PARATHYROID GLANDS 



Anatomy. The parathyroid glands in appearance resemble lymph 

 nodes, and are usually four in number in man. Their diameter varies 

 from three to thirteen millimeters. Their position is also variable. They 

 generally lie dorsal to the thyroid, but may be occasionally enclosed 

 within its connective tissue capsule. This relation, however, is purely 

 topographical; there is no functional similarity. Their blood supply is 

 from the inferior thyroid arteries. 



Histology. Unlike the thyroids, the parathyroids are formed of 

 masses and cords of polygonal epithelial cells, among which numerous 

 blood vessels are interspersed. Colloid-filled follicles are rare; but they 

 multiply in number when the thyroid is removed, and may take over the 

 function of the thyroid in an emergency. 



Physiology. While, therefore, an animal may lose its thyroid glands 

 without necessarily fatal consequences, the removal of the parathyroids 

 is followed by convulsions and death unless parathyroid extract is adminis- 

 tered intravenously. The effects of loss of parathyroids resemble those 

 which follow the use of strychnia or the bite of a mad-dog. There is a 

 marked fall in the calcium content of the blood, followed by cramps and 

 muscular tetany. Apparently calcium salts act as a nerve sedative to 

 prevent acute stimulation of muscular contraction, so that hypopara- 

 thyroidism has been thought to be one of the causes of extreme irrita- 



