THYMUS AND THYROID BODIES. 
635 
from the heart forwards along the internal side of the jugular vein, par vagum and 
carotid arteries. These two anterior portions of the lateral blastema, from the narrow 
portion forwards to the skull, are the lateral portions of the thymus and thyroid, 
which have not yet joined across the middle line. 
In embryos a little further advanced, the two portions of blastema join across 
the trachea in a line extending from the base of the heart to the lower end of the 
larynx, which has now appeared as an oblong oval swelling behind the tongue. 
Previous to, and also contemporaneous with this cross junction, a change has occurred 
in the position of the lungs and of the ductus Cuvieri. 
As the lungs proceed in development, they pass in a direction from behind forwards 
and from within outwards, moving from their original median position to a lateral 
one : they at the same time increase both absolutely and relatively. At the same 
time, a somewhat similar change takes place in the two ductus Cuvieri. They pass 
forward so as to appear to enter the anterior instead of the posterior extremity of the 
auricle, becoming in this way the anterior venae cavae, this change of position being- 
produced apparently by a semi-revolution of the whole heart, coinciding with its 
elongation and the altered arrangement of the bulbus aortae. 
Coincident with this change in the ductus Cuvieri is a corresponding change in 
the position of the lateral masses of the blastema. These pass forward, become 
grouped around the auricles and anterior venae cavae, and join across the middle line 
as already stated ; but a narrow portion, particularly along the left side, still passes 
downwards and backwards along the cardinal veins, which have now become the 
azygos veins. 
While these changes in the veins and blastema have taken place, the lungs have 
increased in size, and their roots have taken up their proper position. In consequence 
of this change in the position of the pulmonary roots and of the ductus Cuvieri, the 
cardinal veins arch over the root of the lungs in the same manner as the azygos vein 
of the adult does. 
At the same time the blastema of opposite sides unites, as has been stated, across 
and in front of the base of the heart and root of the neck. 
Shortly after this period, the posterior part of the blastema, which has now ad- 
vanced, as already stated, from the sides of the chest to the front of the heart, becomes 
separated by a narrow neck from the cervical portion. The posterior part has now 
become the thoracic portion of the thymus, and in the embryo of the Sheep is largest 
on the left side, corresponding in this respect to the large size of the left vena azygos 
and left vena cava at this period. 
The cervical portion of the blastema now begins to exhibit a separation into the 
thyroid and cervical portion of the thymus. This is effected by the absorption of a 
portion of the blastema, of a triangular form, a little behind the larynx, the apex 
looking backwards, the concave base forwards, so that the future thyroid present a 
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