THE THYMUS 



1321 



widest and largest part of the organ and has no distinct separation from the 

 extremities. The inferior extremity is also broad and is known as the base. It 

 rests on the pericardium, ordinarily extending as far caudal at birth as the atrio- 

 ventricular furrow but rarely it may extend as far as the diaphragm. The 

 superior extremity is much elongated and extends into the neck. It is represented 

 by two horns nearly always unequal in size the left being usually the larger. 

 It extends nearly to the thyreoid gland, in some cases reaching it. 



Relations. — Topographically the thymus when well developed is divided into 

 cervical and thoracic parts. 



The cervical portion presents for examination an anterior surface and a posterior surface. 

 The anterior surface is convex and is in relation with the sterno-thyreoid and sterno-cleido- 

 mastoid muscle. The posterior surface is concave and rests medially upon the anterior surface 

 of the trachea, laterally upon the common carotid artery and sometimes on the left side upon the 

 oesophagus. 



The thoracic portion of the thymus is much more important representing four-fifths of the 

 organ (fig. 1075). It presents for examination an anterior, a posterior, and two lateral surfaces. 

 The anterior surface is dorsal to the sternum from which it is separated cephalically by the 

 origin of the sterno-thjTeoid muscle. To a less extent it is in relation with the sterno-clavicular 

 articulation and comes into contact laterally with three or four of the cephalic sterno-costal 

 articulations and lateral to this with the internal mammary artery. The posterior surface is 

 largely concave and is in relation caudally with the pericardium which separates it from the 



Fig. 1075.— Thymus in an Adttlt. Xl (From Toldt's Atlas.) 

 Superior mediastinum "~ — .<! 



Right lung, 

 superior lobe 



Mediastinal . 

 pleura 



Right lung, 

 middle lobe 



Pleural 



cavity — 





sternal end of clavicle 



Thymus, right and 

 ^ left lobes 



■»■•) 



(it- 



i [\ 



Mediastinal 

 pleura 



Left lung 



X^ ^ ... Cardiac 

 f\\ ^ '" pleura 



l\ ^ ■^ Anterior 



mediastinal 

 space 



right atrium and ventricular portion of the aorta and pulmonary artery. The middle part is in 

 relation with the aorta and to the right of this with the superior vena cava. The cephahc part 

 is in relation with the branches of the aorta and superior vena cava. The lateral surfaces are 

 somewhat flattened and are separated from the lungs by the mediastinal pleura. The phrenic 

 nerve on the right side runs in the pleura near the dorsal border of this surface, on the left it is, 

 as a rule, not in direct contact wath the thymus. 



Structure. — The two lateral lobes of which the thymus is composed are rarely of the same 

 size; the right is usually the more strongly developed. They are joined at an obliqiie plane so 

 that the ventral surface of the right is narrow and its dorsal surface broader and the reverse 

 condition is foimd in the left lobe. The two lobes are separated from one another by connective 

 tissue. Rarely the two are joined by a medial portion, isthmus, near the middle or toward the 

 caudal end (fig. 1076). 



Each lobe of the thymus is completely surrounded by a thin delicate connective-tissue 

 capsule from which numerous septa extend through the gland accompanied by the blood-vessels 

 and nerves. The capsule is composed mainly of white fibrous connective tissue with some 

 elastic fibres. It rarely contains much fat in the newborn but the amoimt of fat increases as 

 development and involution proceed. Fibrous prolongations from the capsule may extend 

 from the apices of the lobes to be attached to the cervical fascia in the region of the lateral 

 lobes of the thyreoid gland, acting as suspensory ligaments for the gland. 



The lobes of thymus are divided into numerous small lobules [lobuli thymi] 4 to 11 mm. in 

 diameter. These are of roundish or polyhedral shape with bases toward the surface where thej- 

 show as polygonal areas. The lobules are separated and also bound together by the loose 

 fibrous tissue septa which extend from the capsule. 



Each of the primary lobules of the thymus is divided into a number of secondary lobules or 

 follicles 1 to 2 mm. in diameter. These Ij-mphoid-like masses of tissue are composed of a 

 reticulum containing in its meshes lymphocytes or thymus corpuscles. The tissue is denser 

 near the surface, forming a cortex and passes gradually into a tissue with looser meshed reticulum 

 near the centre, medulla. In the medulla there are nests of concentrically arranged de- 

 generated epithehal cells enclosing a central mass of granular cells containing colloid. These 



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