1156 THE DUCTLESS GLANDS. 
originally separate and distinct rudiments which arise independently of each other—viz. a 
median rudiment and two lateral rudiments. 
The median thyroid rudiment arises as a hypoblastic evagination or tubular diverti- 
culum from the floor or ventral wall of the pharynx (Fig. 28, F.C, p. 35). The point at 
which this occurs is in front of the second visceral arches, at the junction between the 
basal portion of the tongue and that part of the organ which is developed from the 
tuberculum impar (see pp. 36 and 963). The foramen cecum on the dorsum of the 
adult tongue represents the upper persistent part of the median thyroid diverticulum. 
The median thyroid rudiment is at first in the form of a short hypobk: istic outgrowth 
which extends downwards towards the front of the larynx. It rapidly elongates, and its 
terminal extremity bifurcates and comes to lie in front of the upper part of the trachea. 
This bifurcated extremity forms the isthmus of the thyroid body, and probably also 
a portion of each lateral lobe. The portion of the diverticulum which intervenes 
between the foramen cxecum and the isthmus is termed the thyro-glossal duct. It very 
early loses its jumen, and becomes converted into a solid cord of epithelial cells. The 
lower part of this usually persists as the pyramidal lobe, whilst the remainder becomes 
broken up into det: iched parts. Certain of these fragments may persist as accessory 
thyroid glands, and indicate the course pursued by the original thyro-glossal duct. Cases 
in which portions of the thyro-glossal duct not only persist but remain patent occasion- 
ally occur. The rare occurrence of a lingual duct, which extends from the foramen cecum 
downwards through the tongue towards the hyoid boue, is accounted for in this manner. 
The lateral thyroid rudiment of each side arises as a saccular hypoblastic diverticulum 
from the pharyngeal side of the fourth visceral cleft (see p. 34). It comes into relation 
with the lateral aspect of the larynx, and becoming cut off from the cavity of the pharynx, 
it joins with the isthmus or median rudiment to form the greater part of the lateral lobe 
of the thyroid body. 
The thyroid body in its primitive condition and in each of its three parts is epithelial. 
Soon it is invaded by connective tissue and blood-vessels, but the hypoblastie epithelium 
is retained as the cellular lining of its constituent vesicles. 
PARATHYROIDS. 
The parathyroid glands are two minute structures which lie in more or less close 
relation to each lateral lobe of the thyroid body. They are apt to be mistaken for 
accessory thyroids, but in structure they are different. One, more constant in 
position than the other, is situated on the posterior aspect of the cesophagus at 
the level of the lower border of the ericoid cartilage, and in more or less intimate 
relation to the posterior border of the lateral lobe of the thyroid body. The 
second parathyroid body is placed either in close apposition with the lower border 
of the lateral lobe of the thyroid, or on the trachea at a varying distance below it. 
The inferior thyroid artery intervenes between the two parathyroid bodies (Welsh)... 
The parathyroid bodies are composed of epithelial cells, and in structure bear 
a close resemblance to the anterior lobe of the pituitary body. It has been shown 
that the removal of the four parathyroids from the cat is followed by very severe 
symptoms, and in two cases out of three death ensued in the course of a few days 
(Welsh). 
THYMUS GLAND. 
The thymus gland can only be studied to advantage in the later period of foetal 
life or in early childhood. It attains its maximum development towards the end 
of the second year, and from this time on it dwindles away until very little of it 
is left. In the new-born child it is of a pinkish colour, and is composed of two 
lateral lobes, separated by an intervening fissure, which are very seldom of equal 
size. The main portion of the gland is placed within the thorax, but the two lobes 
end above in two blunt prolongations which are carried upwards for a varying 
distance into the neck. 
The thoracic portion of the thymus gland, in its fully-developed condition, is 
placed in the superior and anterior mediastinal spaces, and as a rule it extends 
downwards as far as the level of the fourth costal cartilages. The mediastinal 
pleura and lung are applied to it on either side, whilst the sternum and costal 
cartilages are In 1 close relation to it in front. The deep surface of the thymus is 
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