556 TIIYMUS GLAND. 



the middle line so as to constitute a single gland. It is doubtful 

 whether it performs any especial function in foetal life. 



THYMUS GLAND. 



The Thymus gland,* consists " of a thoracic and a cervical por- 

 tion on each side. The former is situated in the anterior mediasti- 

 num, and the latter is placed in the neck just above the first bone of 

 the sternum, and behind the sterno-hyoidei and sterno-thyroidei 

 muscles." It extends upwards from the fourth rib as high as the 

 thyroid gland, resting upon the pericardium, and separated from the 

 arch of the aorta and great vessels by the thoracic fascia in the 

 chest, and lying on each side of the trachea in the neck. 



Although described usually as a single gland, it consists actually 

 of two lateral, almost symmetrical glands, connected with each 

 other by cellular tissue only, and having no structural communica- 

 tion ; they may therefore be 



Fig. IGS.t "properly called a right and 



left thymus gland." 



Between the second and 

 third months of embryo exist- 

 ence, the thymus is so small as 

 to be only "just perceptible;" 

 and continues' gradually in- 

 creasing with the growth of 

 the fastus until the seventh. At 

 the eighth month it is large ; 

 but, during the ninth, it under- 

 goes a sudden change, assumes 

 a greatly increased size, and 

 at birth weighs 240 grains. 

 After birth it continues to en- 

 large until the expiration of the first year, when it ceases to grow, 

 and gradually diminishes, until at puberty it has almost disappeared. 

 The thymus is a conglomerate gland, being composed of lobules 

 disposed in a spiral form round a central cavity. The lobules are 

 held together by a firm cellular tissue (" reticulated"), and the entire 

 gland is enclosed in a coarse cellular capsule. 



The Lobules are very numerous, and vary in size from that of the 



* In the description of this gland I have adhered closely to the history of it given 

 by our great authority on this subject, Sir Astlcy Cooper, in his beautiful monograph 

 " On the Anatomy of the Thymus Gland," 1832. 



t A section of the thymus gland at the eighth month, showing its anatomy. This 

 figure, and the succeeding, were drawn from two of Sir Astley Cooper's beautiful pre- 

 parations, with the kind permission of their possessor. The references were made by 

 Sir Astley's own hand. 1. The cervical portions of the gland ; the independence of 

 the two lateral glands is well marked. 2. Secretory cells seen upon the cut surface of 

 the section ; these are observed in all parts of the section. 3, 3. The pores or openings 

 of the secretory cells and pouches, they are seen covering the whole internal surface of 

 the great central cavity or reservoir. The continuity of the reservoir in the lower or 

 thoracic portion of the gland, with the cervical portion, is seen in the figure. 



