1088 



THYMUS GLAND. 



Fig. 718. 



Thymus, <?., of human fcetus at third month. (After 

 Sir A. Cooper.) 



month (fig. 720.) has undergone a sudden 

 change, becomes of great size, and is said to 

 weigh half an ounce, from which circumstance, 

 however, on account of the cavities which it 



Fig. 719. 



TJiymns, fyc., of human foetus at fifth month. 

 Sir A. Cooper.) 



(Afte 



contains, and the varieties to which it is sub- 

 ject, no judgment of its bulk can be formed. 

 It increases after birth, and continues large to 

 the first year, when it slowly disappears to the 

 time of puberty; and in after age it ceases to 

 have cavities, and becomes a body of very 

 small dimensions." 



He next notices the following varieties in 

 configuration : " Although the gland is usu- 

 ally double, and the one side united to the 

 other by cellular membrane only, yet it some- 

 times happens that a third thoracic lobe exists, 

 which appears to join one lobe with the other, 

 but which allows, under a careful dissection, 

 of their being separated. There are also two 

 other varieties I have seen ; the first is the 

 vena innominata passing through the gland, 

 and the second, the same vein placed anteriorly 

 to the cervical lobes. Indeed, I scarcely find 

 two organs alike in form ; sometimes they are 

 round, whilst others are of great length, and 

 are so thin that the serpentine disposition of 

 their lobes may be seen without dissection. 

 The left gland is often larger than the right; 

 but even in this respect so much variety is 

 observable, that it appears if the bulk of the 



Fig. 720. 



Ttiymus, fyc.,of human fcettts at ninth month. (After 

 Sir A. Cooper.) 



whole be the same, that it is of little import- 

 ance which may be of the greater magnitude, 

 the right or left gland, as its secretion will be 

 equally abundant." 



The relative situation of the thymus gland 

 to the adjacent parts is described as follows : 

 " In cutting through the sternum in its 

 long axis, and then separating its two lateral 

 portions, so as to give a good view of the 

 mediastinum, the thymus gland appears situ- 

 ated behind the first and part of the second 

 bone of the sternum ; and posteriorly to the 

 origins of the sterno-hyoidei and sterno-thy- 

 roidei muscles. It reaches more than half 

 way down the sternum at birth, viz. to the 

 fourth rib, and extends from thence into the 

 neck near to the thyroid gland. It is con- 

 nected to the sternum and origins of the 

 sterno-hyoidei and sterno-thyroidei muscles by 

 cellular tissue ; it adheres strongly, by a coarse 

 cellular membrane, to the pericardium ; an- 

 teriorly and laterally the internal mammary 

 arteries and veins take their course. The 

 reflection of the pleura descending from the 

 cartilages of the ribs on each side, and con- 

 tinued to the fore part of the pericardium 

 forming the anterior mediastinum, makes its 

 lateral boundaries, and separates it from the 

 lungs; posteriorly it rests upon the vena in- 

 nominata, and upon the fascia of the thorax, 

 which descends from the sternum and first rib 

 to the curvature of the aorta, and to the three 

 large vessels which spring from it." " Such, 

 then, is the relative situation of the gland in 

 the chest. In the dissection of the cervical 



