518 SUTHERLAND SIMPSON 



pends upon the general condition of the animal at the time of the experi- 

 ment, j 



Erdheim also noted changes in the enamel of the teeth when the 

 animal lived sufficiently long to show these. He ascribed this to a lack of 

 calcium in the growing tissue, hut this aspect of his work will be considered 



later. 



Studies on the Comparative Anatomy of the Parathyroids. Follow- 

 ing the publication of Kohn's paper, a great many investigators made care- 

 ful studies of the comparative anatomy of the thyroid and parathyroid 

 glands in many different species. Their anatomical relations were inves- 

 tigated in the following animals : dog, cat, man, rabbit, horse, ox, guinea 

 pig, monkey, fox, goat, pig, seal, bat, rat, mouse, and some birds. Most 

 observers agree that the parathyroids are four in number., two internal and 

 two external glands, except in some species, such as the rat, in which only 

 two are present, So constant, indeed, was the number supposed to be, 

 that Groschuff made the suggestion that mammals might be classified in 

 two groups, according to whether the species possessed two or four para- 

 thyroids. Occasionally an accessory gland was reported, but four para- 

 thyroids were generally regarded as the greatest number ever present. It 

 is important to keep this in mind when one considers the results of extir- 

 pation experiments reported during this period. 



Rogers and Ferguson (1906) examined microscopically 253 "suspi- 

 cious pieces" of tissue removed from human adults, infants, dogs, and 

 horses, and found only 36 per cent of these bodies to be parathyroids, 

 while 42 per cent consisted of thyroid tissue, in spite of the fact that these 

 nodules were selected "because of a resemblance to parathyroid." The 

 remaining 22 per cent consisted of lymphatic glands, thymus, vascular 

 connective tissue, and mucous gland. 



Moussu (&) (1896) had noted the occurrence of supplementary thy- 

 roids in the dog, but Jeandelize (1903) doubted their existence. 



Schaper (1895) stated that in the sheep he has very often found 

 nodules of a parathyroid structure near the bifurcation of the common 

 carotid, and others two, three or more along the course of this artery. 

 Jeandelize lias confirmed this in the sheep. 



Forsyth (?;) (1908) made an extended research into this subject, using 

 forty-two mammals and thirty-five birds of many different species obtained 

 from the Zoological Gardens in London. The method of procedure was 

 as follows: Every glandular body was removed from the neck and sub- 

 jected to careful microscopic examination. The thyroid itself, if small, 

 was examined by serial section, but, if large, was cut into thin slices, the 

 surfaces of each slice being carefully scrutinized with a lens and all sus- 

 picious looking pieces retained for microscopic examination. "In brief, 

 the guiding principle followed was to regard every gland as a possible para- 

 thyroid, and none has been identified except by its microscopical features." 



