THE CEEVICAL THYMUS VESTIGES. 1351 



1 It is impossible to say what should be the normal dimensions of the gland at the 

 various ages. In some new-born babes it weighs as little as 2 or 3 gm., in others 

 ( as much as 15-17 gm. At puberty it may be difficult to find, or may weigh as 

 much as 40 gm. After the age of fifty it may require careful dissection to dis- 

 cover, or may be quite large. When large it fills all the space available between 

 the pleural sacs laterally, the back of the sternum in front, and the pericardium 

 and great vessels behind; when small it is embedded in fat and fibrous tissue. 

 The shape of the gland varies with its size and the age of the individual. In 

 infants with short thoraces it is broad and squat; in adults with long thoraces 

 it is drawn out into two finger-like strands. The details of its shape are deter- 

 mined by its size and by the structures upon which it is moulded, viz., the peri- 

 cardium and the great vessels of the superior mediastinum and the root of the 

 neck. One or both of its lobes may be connected by a strand of fibrous tissue 

 to the tunica propria of the thyreoid gland. 



Blood and Lymph Vessels. The blood supply of the thymus is effected through inconstant 

 branches of the inferior thyreoid and internal mammary arteries. Its veins are irregular and 

 join the inferior thyreoid, internal mammary, and innominate veins. Its lymph vessels are 

 large and pass to glands close to the organ. 



Its nerves are minute and are derived from the vagus and sympathetic. The branches of 



' the vagus descend directly to the thymus from about the level of the thyreoid cartilage ; the 



sympathetic fibres run with the blood-vessels. The fibrous capsule of the thymus receives small 



irregular branches from the phrenic nerves, but these do not supply the gland tissue in any way. 



Structure. The thymus is invested by a 



a , , 1--IJ A. i Groove tor ^t*^^j^^ Groovft for Ipff- 



fibrous Capsule which sends septa into Its sub- pulmonary artery JM Rpifc" innominate vein 

 5 tance to divide it into lobules The lobules are Mediastinal $M jf^kg* Groove for . 



i similarly divided into follicles (secondary lobules) surfaceX J^^H |Q-- fl^il^ vena ? ava 

 ibout 1'5 mm. in diameter. Each follicle consists superior 



of a medulla not quite completely surrounded by 

 i cortex. In a general way, the structure of the 

 iortex resembles a lymph gland, but the reticulum, 

 instead of being fibrous, is syncytiaL The spaces 

 )f the reticulum are crowded with lymphocytes, 

 out there are no germinal centres. The medulla 

 resembles the cortex, but the reticulum is coarser 

 md contains cell nests, the concentric corpuscles 

 )f Hassall. As age advances the thymus under- Pericardial surface 



*oes involution. This process is marked by an FIG. 1061. DEEP SURFACE OF THYMUS, TAKEN 

 .ncrease of fibrous tissue and a reduced cellularity. FROM A FCETUS HARDENED BY FORMALIN - 



Tlie number of lymphocytes and of concentric INJECTION. 



corpuscles varies with nutrition. 



Development. As has been stated, there are in reality two thymus glands, a right and a 

 eft ; they arise from the ventral diverticula of the third pharyngeal pouches. The first 

 ndications of the developing glands, cylindrical elongations of the diverticula, are present in 



j 3-mm. embryos ; the walls of the cylinders, more particularly their dorsal parts, soon thicken. 

 Uoincidently the necks of the pharyngeal pouches become constricted to form the pharyngo- 

 jranchial ducts III. These soon disappear when the thymus rudiments lose all connexion 

 vith the pharynx. At this time the upper parts of the rudiments still have a lumen ; the 

 ower parts are solid. Soon the lumen vanishes ; the solid parts thicken and the developing 

 ihymus migrates caudaiwards to reach the pericardium at the 15-rnni. stage. As a result of the 

 uigration the upper part gets drawn out and finally disappears. It is in this process that 

 )ara thyreoid III. is involved. It is attached to the upper part of the migrating thymus, the part 

 vhich disappears. The relative time of this disappearance determines the permanent level of 

 )arathyreoid III., for until it happens that gland is dragged in the wake of the thymus (see 

 Parathyreoids, Development of). Sometimes a small detached mass of thymus formative tissue 

 nay persist beside parathyreoid III., and may differentiate to form an Accessory Cervical 

 Miymus III. 



During migration and after, the cells continue to proliferate and the thymus rudiment 

 ncreases in mass. At the 40-mm. stage lymphocytes begin to appear in it. Differentiation 

 >f cortex and medulla is visible at the 45 mm. stage. The details of the process of thymic 

 listogenesis are undetermined. It is believed that the syncytial reticulum and concentric 

 orpuscles are of entodermal origin, but whether the lymphocytes arise in situ or are immigrants 

 : s unknown. 



(iv.) THE CERVICAL THYMUS VESTIGES. 



Small masses of thymus tissue are frequently found in close relation to parathyreoid s 

 -V. They are developed from the ventral diverticula of the fourth pharyngeal pouches 

 11 a manner generally similar to that in which the main thymus gland develops. Not 



86 a 



