THE THYMUS GLAND. 297 



perfectly distinct from each other. Each vesicle consists of a simple 

 basement membrane (fig. 199 t), with a single epithelial layer of cells 

 (c) lining its inner surface ; at least in the foetus and young subject, for 

 it would appear that the cells for the most part becoma detached in the 

 progress of growth. The fluid coagulates by the action of heat or of 

 alcohol, preserving, however, its transparency. 



One of the most frequent pathological changes to which the thyroid 

 body is subject consists in the accumulation within its vesicles of a 

 gelatinous-looking substance (colloid) : this may occur without giving 

 rise to very great enlargement of these vesicles, but in certain forms 

 of goitre it distends them to an enormous degree. 



Vessels and nerves. — The artcricf: of the thyi-oid body are the superior and 

 inferior thyi-oids of each side, to which is sometimes added a fifth vessel, the 

 thijroidca ima of Neubauer and Erdmann. The arteries are remarkable for their 

 large relative size, and for their frequent and large anastomoses ; they teiininate 

 in a capillary network, uifon the outside of the closed vesicles. The vrina. which 

 aa'e also large, ultimately form plexuses on the surface, from which a superior, 

 middle, and inferior thjToid vein are formed on each side. The superior and 

 middle thyi'oid veins open into the internal jugular ; the inferior veins issue 

 from a plexus formed in front of the trachea, and open on the right side into 

 the superior cava, and on the left into the brachio-cephalic vein. The ////iij/hnticx 

 of the thyroid body form numerous and large anastomosing ti-uuks. both at the 

 sm-face of the organ and throughout its sulDstance : they originate, according to 

 the observations of Frey, in the connective tissue which unites the gland- vesicles, 

 with the cavity of which they appear not to be in communication. 



The ncrrcfi are derived from tire middle and inferior cervical ganglia of the 

 symijathetic. They accompany the blood-vessels ; and have here and there 

 ganglion-cells in their course ; theii" mode of ending is unknown. 



Development. — Remak states that the thjToid body is developed from the 

 anterior wall of the pharjaix. In a human embiyo at the thu-d month. KoUiker 

 found the thjToid body consisting of isolated vesicles, with rounded cells in their 

 interior. The multiplication of these vesicles takes place, according to KoUiker. 

 either by constriction and subsequent division of one vesicle into two, or by a 

 process of gemmation. The transverse part of the gland is said to be develoj^ed 

 subsequently to the two lateral lobes. In the foetus, and dm-ing early infancy. 

 this organ is relatively larger than in after-life ; its proportion to the weight of 

 the body in the new-born infant being that of 1 to 240 or 400. whilst at the end 

 of three weeks it becomes only 1 to 11 GO. and in the adult 1 to 1800 (&ause). 

 In advanced life the thjToid body is liable to become indurated, and frequently 

 contains earthy deposit ; its vesicles also attain a very large size. 



THE THYI>I'JS GliAND. 



The iltymus gland or lodij is a temporary organ which reaches its 

 greatest size at about the end of the second year of life, after which period 

 it ceases to grow, and is gradually reduced to a mere vestige. Its function 

 is not fully understood, althoug'h it is probable that it is in some way 

 connected with the elaboration of the blood in infancy. "When examined 

 in its mature state in an infant under two years of age, it appears as a 

 narrow elongated glandalar-looking body, situated partly in the thorax, 

 and partly in the lower region of the neck : below, it lies in the 

 anterior mediastinal space, close behind the sternirm, and in front ot 

 the great vessels and pericardium ; above, it extends upwards upon 

 the trachea in the neck. Its colour is greyish, with a pinkish 

 tinge ; its consistence is soft and pulpy, and its surface appears dis- 

 tinctly lobulated. It consists of two kiieral Johes, which touch eacli 



