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 oesophageal branches of the broncho-oesophageal 

 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.^ 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 j miction 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. thyreoidese) 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. 5 cm.). The greatest height averages a little more than an 

 inch (ca. 2.7 cm.), and the greatest width a little less than an inch (ca. 1.5-2 cm.). The average 

 weight is about half an ounce (ca. 15 gm.). The right lobe is commonly in contact with the crico- 

 pharyngeus or crico-thyroideus, but may be about a finger's breadth from the larjmx. The left 

 lobe may be an inch or more (ca. 4 cm.) 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 u-- 

 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 rmgs. 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 



1 It 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. 



