184 



Mr. Callender on the Thyroid Body, 



[1867. 



subclavian arteries. At first, he states, no isthmus is observed (in sheep), 

 but presently lateral masses join across and in front of the base of the 

 heart and root of the neck. Then the thyroid separates from the thymus, 

 from which it differs in not being divided into lobules. Simon* has no 

 reason for believing that its origin has any particular relation to that of 

 the thymus. Undoubtedly, he adds, there is a period when it is im- 

 possible to say how much of the unshaped blastema of the neck belongs 

 to one organ, how much to another, but so soon as the microscope can 

 discover the first traces of their development, it likewise affords unques- 

 tionable evidence of their distinctness, and shows each as separate in itself, 

 and as peculiar in structure, as at any later period of growth. 



Handfield Jones f found in a foetal sheep two inches long that the thy- 

 roid presented the usual appearance ; it possessed an isthmus, and in a 

 human foetus of four months and a half, the isthmus appeared of the same 

 standing as the lateral masses. 



The absence of an isthmus in an entire class» that of birds, the observa- 

 tions of Gray on the formation of this body in the chick, and the sugges- 

 tions of Meckel, Cruveilhier and others, countenance the supposition that 

 the thyroid is developed from two lateral masses. 



There are no reliable observations respecting the development of the 

 pyramid, but Haller and Arnold have hazarded the opinion that it is pro- 

 bably the duct of the thyroid during foetal life. 



In describing the following dissections, I may state that the specimens 

 were examined as they came into my possession without selection, save for 

 age, so that they may be assumed to represent very fairly the conditions 

 commonly existing during foetal life. 



In a human foetus, measuring in length eight-tenths of an inch (be- 

 tween the seventh and eighth week), the thyroid is a body of a pale yel- 

 lowish colour, lying across the front of the trachea, just below the mass 

 out of which the cartilages of the larynx are being developed. It is 

 closely connected with the trachea and with the lower edge of the larynx, 

 either of which would be torn in endeavouring to remove it, but the thy- 

 roid is easily uncovered by stripping off the parts superficial to it, and 

 has no connexion with these or with the thymus (fig. 1)$. Although 

 consisting at this period of but one piece, the thyroid is deeply notched, 

 and thus looks as though made up of three distinct lobes, one sometimes 

 bifid, in the centre, and this is the smallest, and another on either side 

 elongated and inclined upwards by the side of the larynx. Similar divi- 

 sions are seen in a foetus measuring two inches and eight-tenths in length 

 (fig. 2)§, the thyroid consisting of three lobes, one being central, a 



* Essay on the Thymus Gland, 1845. 



t Cyclopaedia of Anatomy and Physiology, Art. Thyroid, 



j This specimen is in the Museum of St. Bartholomew's Hospital. 



§ See also specimen No. 1. 



