ES —  e S C 
LA a 
THYMUS GLAND. 1157 
moulded upon the pericardium and upon the vessels in the front part of the 
superior mediastinum. Thus, when it is hardened im situ and removed, it 
presents on its posterior surface a deep pericardial concavity, with impressions 
on the upper part of this 
hollow corresponding to 
the pulmonary artery and 
vena cava superior (Fig. 
791). Above the peri- 
cardial surface deep 
grooves indicate the 
intimate manner in 
which it is adapted to 
the two innominate veins. 
Above the left innomi- isamnnatoncicr aed 
nate vein the two lobes NG 
of the thymus ascend Se Eth ee 
to an unequal height 
into the neck. They 
are placed in front of 
the trachea, and extend 
outwards so as to over- 
lap the termination of 
the imnominate artery 
on the right side and 
the left common carotid 
artery on the left side. 
One or other of the lobes 
may rise as high as the 
thyroid gland, but as a 
rule both fall somewhat 
short of this organ, and 
the lobe which ascends 
highest is usually 
attached to the corre- 
sponding lateral lobe of 
the thyroid body by 
strand of connective 
Lateral lobe of thyroid 
body 
Isthinus of thyroid 
- | body 
~ Common carotid artery 
Internal jugular vein~_ 
Vagus nerve~_|\ 
Common carotid artery~_\ ~ - Internal jugular vein 
Band connecting 
— thymus with thyroid 
_ Vagus nerve 
Subclavian vein —— 
aul * i eit, aR 
Se ae y ? go Subclavian vein 
> q Innominate vein 
Innominate 
vein 
PERICARDiyy, 
"i 
J 
Fic. 790.—THyMus GLAND IN A FULL-TIME F@TUS HARDENED BY 
tissue. FORMALIN-INJECTION. 
After the second year 
the thymus gland remains stationary, or it begins slowly to diminish in size, 
but when puberty is reached a rapid degeneration sets in. Its lobules become 
infiltrated with fat and loose strands of connective tissue. Waldeyer has shown, 
however, that throughout the whole of life it not only retains something of its 
old form, but also that the degeneration is never complete. Preserved within its 
substance (either uniformly diffused through it or in distinct masses)’ there may 
always be found remains of the original thy mus-tissue. 
Blood-vessels and Nerves.—The arteries which carry blood to the thymus come from the 
inferior thyroid, the internal mammary, and perhaps also from other sources. The veins join 
the neighbouring venous trunks—viz. the inferior thyroid, the internal mammary, and the two 
innominate veins. 
The nerves to the thymus are derived from the vagus and sympathetic trunks. 
The lymphatic vessels are of large size, and accompany the blood-vessels. 
Structure of the Thymus.—The thymus is composed of a large number of small 
polyhedral lobules. The sheath which envelops the organ sends off from its deep surface 
fine partitions or septa which pass into the gland and separate the different lobules from 
each other. 
Each lobule is composed of clusters of lymphoid follicles, with a small amount of 
delicate connective tissue intervening between them. A follicle consists of an outer 
cortical and an inner or central medullary portion. Both are formed of adenoid tissue, 
Subclavian artery 
