1316 THE SKIN, MAMMARY GLANDS AND DUCTLESS GLANDS 



The connective tissue which covers the surface of the gland forming for it a 

 capsule, may be divided into two layers, superficial and deep. 



The superficial layer intimately connected with and derived from the fascia colli as pointed 

 out above has an important fimction in supporting and fixing the gland. This layer is in some 

 cases thin and transparent; in other cases it is very tough and thick. It is connected by loose 

 areolar tissue with the thin deep layer of the capsule. Between these two layers the larger 

 vessels run for a space before entering the gland and the veins, particularly, form here con- 

 siderable plexuses. 



From the deeper layer of the capsule numerous trabeculse and septa carrying 

 blood-vessels, lymphatics, and nerves pass into the gland and imperfectly separate 

 its parenchyma into irregular masses of variable size, the lobules [lobuli]. Each 

 lobule is composed of a number of closed, non-communicating, irregular, spherical, 

 ovoid, or sometimes branched alveoli, acini or vesicles, varying in size from .045 

 to .22 mm. in diameter and separated and bound together by a vascular connect- 

 ive tissue continuous with that surrounding the lobules and with that of the cap- 



FiG. 1071. — Arteries of the Thyreoid Gland, Anterior View. 1. Lateral lobe; l' 

 pyramidal lobe; 2, trachea; 3, thyreoid cartilage; 4, crico-thyreoid membrane; 5, hyo-thyreoid 

 membrane; 6, 7, 8, 9, inferior thjTeoid artery and branches; 10, 11, 12, 13, 14, 15, superior; 

 thyreoid artery and branches; 16, thyreoidea ima. (Testut and Jacob.) 



..3 



1& 2 



sule. The vesicles are filled with a yellowish viscous fluid, known as colloid, 

 the secretion of the epithelial cells. 



The vesicles are lined with a single layer of epithelial cells of a fairly uniform cuboidal or 

 columnar shape, becoming flattened in distended vesicles and in old age. The cells are not 

 supported by a basement membrane but are in close relation with connective tissue and 

 capillary blood-vessels. An extremely rich lymphatic network surrounds the vesicles and the 

 lymph-ves.sel8 come into intimate relation with the cells. Through these vessels the secretion 

 is conveyed from the gland to the general circulation. 



Blood-vessels. — The thyreoid gland has an extremely abundant blood- 

 supply. The arteries are usuall}'- four in number but occasionally five (figs. 

 1071-1073). 



The superior thyreoid arteries divide into two, three, or more main branches which reach the 

 gland near the apex of the lateral lolxis and supjjly mainly the ventral and medial surfaces of the 

 cephalic portion of the lobes (fig. 1071). There is usually also a dorsal l)ranch, which anasto- 

 moses witli a brancli from the inferior fhyreoid. One of the ventral branches frequently con- 

 nects along the (;ophalic border of the isthmus with its follow of the opposite side. The inferior 

 thyreoid arteries break up into two or three main branches, occasionally into many fine twigs, 

 which reach the dorsal surface of the lateral lobes near the caudolateral borders and supply 



