THE THYROID GLAND 535 
on the walls of the alveoli. Here the blood is arterialized, and is returned to the 
heart by the pulmonary veins. The bronchial arteries are relatively small vessels 
which carry arterial blood for the nutrition of the lungs. The branches of these 
arteries accompany the bronchial ramifications as far as the alveolar ducts, but 
do not extend to the alveoli. The cesophageal branches of the broncho-cesophageal 
and gastric arteries also supply pulmonary branches (in the horse) which reach the 
lung by way of the ligament of the latter. These branches vary much in size, and 
ramify chiefly in the subpleural tissue of the basal part of the lungs.1_ The lymph 
vessels are numerous, and are arranged in two sets. The superficial set forms 
close networks in and under the pleura, while the deep set accompanies the bronchi 
and pulmonary vessels. Most of them converge to the root of the lung and enter 
the bronchial lymph glands, but some go to the mediastinal lymph glands. The 
pulmonary nerves come from the vagus and sympathetic nerves. They enter at 
the hilus and supply branches to the bronchial arteries and the air-tubes. 
THE THYROID GLAND 
The thyroid gland (Glandula thyreoidea) is situated on the anterior part of the 
trachea, to which it is loosely attached. It is a very vascular ductless gland, and 
is firm in texture and dark red-brown in color. It consists of two lateral lobes and 
a very narrow connecting isthmus (Figs. 332, 649). 
The lateral lobes (Lobus dexter, sinister) are situated on each side of the trachea 
near to or in contact with the larynx. Their position is indicated approximately 
by the angle of junction of the jugular and external maxillary veins. Each is oval 
in outline and is about as large as a plum of medium size. The superficial surface 
is convex and is covered by the cervical angle of the parotid gland and the sterno- 
cephalicus and omo-hyoideus. The deep surface is adapted to the trachea. About 
the lobe are the anterior cervical lymph glands. The anterior pole is large and 
rounded, while the posterior is smaller and often tapers to a tail-like process which 
is continuous with the isthmus. 
The isthmus (Isthmus gl. thyreoidex) extends across the ventral face of the 
trachea, connecting the two lobes. It is usually extremely narrow in the adult 
horse and is often reduced in great part to a small strand of fibrous tissue. 
The lateral lobes are very variable in size and position, and are usually unsymmetrical. The 
average length is about two inches (ca.5cm.). The greatest height averages a little more than an 
inch (ca. 2.7 em.), and the greatest width a little less than an inch (ca. 1.5-2 em.). The average 
weight is about half an ounce (ca. 15 gm.). The right lobe is commonly in contact with the crico- 
peeeyngeus or erico-thyroideus, but may be about a finger’s breadth from the larynx. The left 
obe may be an inch or more (ca. 4 em.) from the larynx, but in some cases is in contact with it. 
The position varies considerably in the dorso-ventral direction also, irrespective of the ventral dis- 
placement noted in enlargement of the gland. The most common form is oval, with a smaller 
posterior pole which tapers to the isthmus. But the isthmus is sometimes connected with the 
middle of the ventral border of one lobe or both, so that the two poles are about alike. Many ir- 
regularities may be observed. Cases occur in which a variable posterior process is connected with 
the body of the gland by a sort of neck. A complete glandular isthmus Is not rare, although it is 
usually very narrow and may easily escape notice; in quite exceptional cases it may be 6-8 mm. 
in width. Its connection with the lateral lobes and crossing of the trachea are quite variable. 
Most often it is connected with the posterior poles and passes almost directly between the two. 
But it may run backward and cross much further back; in one case (personal observation) the 
crossing was at the space between the ninth and tenth tracheal rings. In the foal the isthmus is 
relatively large and entirely glandular. In the ass and mule there is almost always a well-developed 
isthmus. Extending from it there is sometimes a narrow band of thyroid tissue which reaches to 
the body of the hyoid bone; this is the lobus pyramidalis. 
Structure.—The gland is enveloped by a thin fibro-elastic capsule, from which 
trabecule pass into the substance of the organ. The gland tissue is compact and 
‘Tt must not be inferred from the necessarily brief account here given that the two sets of 
vessels are quite distinct. On the contrary, competent observers state that numerous small 
branches of the bronchial arteries anastomose with pulmonary vessels. 
