THVMUS GLAND. 621 



two lateral, almost symmetrical glands, connected with each other by 

 areolar tissue only, and having no structural communication: they 

 may therefore be " properly called a right and left thymus gland." 



Between the second and third months of embryo existence, the 

 thymus is so small as to be only " just perceptible ;" and continues 

 gradually increasing with the growth of the foetus until the seventh. 

 At the eighth month it is large ; but, during the ninth, it undergoes a 

 sudden change, assumes a greatly increased size, and at birth weighs 

 240 grains. After birth it continues to enlarge 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 dis- 

 posed in a spiral form around a central cavity. The lobules are held 

 together by a firm areolar tissue (" reticulated "), and the entire gland 

 is enclosed in a coarse areolar capsule. 



Fig. 183.* 



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

 head of a pin to a moderate-sized pea. Each lobule contains in its 

 interior a small cavity, or " secretory cell" and several of these cells 



* A section of the thymus gland at the eighth month, shewing its anatomy. 

 This figure, and the succeeding, were drawn from two of Sir Astley Cooper's 

 beautiful preparations, with the kind permission of their possessor. The refer- 

 ences 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. Secre- 

 tory 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 dispersed upon 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. 



