1158 THE DUCTLESS GLANDS. 
but in the cortex the lymphoid cells are packed much more closely, whilst in the medulla 
the retiform matrix is coarser and the lymphoid cells less numerous. Further, the medulla 
contains the concentric corpuscles of Hassall. 
Groove for : . 
: These are curious bodies, composed of flattened 
pulmonary artery /~ 
~ Groove for left 
AW’ innominate vein 
Rreiastinal Groovetion epithelial cells arranged concentrically around 
surface Meee a granular nucleated corpuscle. A special 
.4:,,, developmental interest is attached to these 
Mediastinal ; 
surface concentric corpuscles. 
The blood-vessels form a fine plexus around 
the various follicles, and from this capillaries 
penetrate into the central medulla. 
< Development of the Thymus Body.— 
bes - The thymus gland is derived from the hypo- 
Pericardial surface blastic lining of the pharynx. It takes origin 
as a tubular diverticulum from the dorsal 
Fic. 791.—DrErp SurRFACE OF THYMUS GLAND, i ae 
Gon A -Rierds HARDENED By Por. Patt Ot the pharyngeal saspeet soi third 
MALIN-INJECTION. visceral cleft on each side (see p. 34). This 
diverticulum has thick epithelial walls, and it 
grows by rapid proliferation of its cells. It extends downwards on the side of the trachea 
towards the pericardium, and coming into contact with the corresponding hypoblastic 
evagination of the opposite side, the two lobes of the organ are formed—one from each 
lateral diverticulum. The narrow upper part of the outgrowth remains for a time 
tubular, and connected with the pharyngeal cleft from which it originates. Ultimately 
this connexion is broken through, and the expanded lower end sends out solid bud-like 
branches after the manner of an acinous gland. 
Originally, therefore, the thymus gland is epithelial in its structure. Soon, how- 
ever, it becomes invaded by connective tissue and quantities of adenoid tissue, so that 
ultimately it is transformed into the lymphoid organ characteristic of the child. 
The remains of the original epithelial constituents are to be seen in the concentric 
corpuscles of Hassall. 
THE CAROTID AND COCCYGEAL BODIES. 
The carotid gland is a minute oval reddish-brown body placed on the deep 
aspect of the common carotid artery at the point where it bifurcates into its two 
terminal branches. It is closely connected with the sympathetic nerve filaments 
which twine around the carotid vessels, and numerous minute arterial twigs 
enter it. In structure the carotid body is composed of nodular masses of poly- 
hedral epithelial-like cells, separated from each other by strands of connective 
tissue. Wide thin-walled tortuous capillaries are brought into intimate relation 
with the cells. 
The coccygeal gland is a small body placed in front of the tip of the coceyx. 
Branches from the middle sacral artery enter it. Its structure is very similar to 
that of the carotid body. 
