536 RESPIRATORY SYSTEM OF THE HORSE 
consists of lobules (Lobuli gl. thyreoidez), which are embedded in a stroma of 
strands of fibrous tissue. The stroma contains numerous vessels. The lobules 
consist of non-communicating alveoli or follicles of varying form and size. The 
alveoli are lined by cubical or columnar epithelium, and contain a viscid colloid 
substance. 
Vessels and Nerves.—The arteries are relatively very large; there are usually 
two thyroid arteries, which arise from the carotid; their branches enter the gland 
chiefly at or near the ends and the dorsal border. The veins are also large; they 
go to the jugular vein; the largest one leaves the gland at its posterior pole. The 
lymph vessels go to the cervical lymph glands. The nerves are derived from the 
sympathetic system. 
Nodules of thyroid tissue of variable size, sometimes as large as a pea, may be 
found near the anterior extremity of the lateral lobes, on the course of the anterior 
thyroid artery or on the trachea, even at a considerable distance from the thyroid 
region. They are termed accessory thyroids (Glandule thyreoidex accessorize). 
The parathyroids are small glandular bodies found in the thyroid region. 
They are paler and not so dense as the thyroid tissue, but often cannot be differen- 
tiated by their naked-eye appearance. In the horse there is usually only one on 
each side in the connective tissue over the dorsal border or anterior extremity of the 
lateral lobe of the thyroid. One may be embedded in the deep face of the lateral 
lobe of the thyroid. They resemble in structure embryonic thyroid tissue. 
THE THYMUS GLAND 
The thymus is a ductless gland which has a close resemblance to the lymphoid 
tissues. It is well developed only in late feetal tife and for a few months after birth. 
After this it undergoes rapid atrophy, fatty infiltration, and amyloid degeneration, 
so that in the adult it is usually represented by a thin remnant in the ventral part 
of the anterior mediastinum or has entirely disappeared. In the new-born foal 
it is of a grayish-pink color, and consists of right and left lobes (Lobus dexter, 
sinister). The greater part of the gland is situated in the anterior mediastinum 
ventral to the trachea and large vessels, but the two lobes are continued into the 
neck by a chain of lobules which lie on the trachea along the course of the carotid 
artery, extending sometimes as far as the thyroid gland.t 
Structure.—The thymus is enclosed in a thin and loose fibro-elastic capsule, 
and consists of secondary lobules (Lobuli thymi), of varying shape and size, held to- 
gether by areolar tissue and fat, and connected with a strand of connective tissue 
termed the tractus centralis. These lobules are subdivided into polyhedral primary 
lobules, which are composed of lymphoid tissue, and present a dark, vascular cor- 
tex, and a lighter medulla that contains small masses of concentrically arranged, 
flattened cells; these are known as concentric corpuscles. 
Vessels and Nerves.—The arteries are derived from the internal thoracic and 
common carotid. The nerves come from the sympathetic and vagus. 
1The cervical part of the gland is very variable. The thoracic lobe of one side may have 
no cervical continuation; the other lobe may then give off a single prolongation which bifureates. 
The thoracic part in the new-born foal occupies most of the space which is later taken up by the 
apices of the lungs. Its two lobes are in contact, and its deep surface is molded on the anterior 
part of the pericardium and the large vessels. In exceptional cases a considerable remnant of the 
thymus is present in adult or even old subjects; this is usually true only of the thoracic part, but 
is may extend into the neck a short distance; but a large lymph gland must not be mistaken for 
thymus. 
