CHAP. XXXV.] MR. SIMON'S RESEARCHES. 



521 



vary considerably in size, sometimes the right and sometimes the 

 left being the largest. 



The organ rests upon the front of the aortic arch and large 

 arteries rising from it, and also on the left vena innominata. It is 

 covered by the sternum, and at birth reaches down to the fourth 

 costal cartilage. It extends upwards into the neck, as high as the 

 thyroid body, and lies upon the front and side of the trachea. 



The researches of Sir Astley Cooper, and more recently those of 

 Mr. Simon, show that this organ consists essentially of an elon- 

 gated tube, from all sides of which extend numerous small follicles 

 or sacculi, which pour their contents into the central cavity. Sir 

 Astley Cooper unravelled the gland, and, by having previously in- 

 jected the central cavity and follicles with alcohol, or coloured 

 size, was enabled to make. out their relations and arrangement ; 

 although, by these processes, it is probable that he distended the 

 central cavity to a greater extent than natural, and was thus led 

 to look upon it as much more extensive than it was subsequently 

 proved to be by the conclusive observations of our friend Mr. 

 Simon, which are published in his well-known essay. 



The latter excellent observer, carefully watched the development 

 of this gland, and thus was the first to make out accurately its 

 anatomy. 



It is probable that it first Fig. 250. 



arises from a row of cells 

 arranged in linear series > 

 which coalesce, and thus be- 

 come converted into a nar- 

 row tube. The wall of this 

 tube then bulges at intervals, 

 and vesicular cavities are 

 gradually formed. These ve- 

 sicular dilatations are much 

 more abundant in some 

 situations than in others; 

 and the primary offset di- 

 vides in a dichotomous or 

 quarternary manner, until, 

 from the number and irre- 

 gular distribution of these 



a. Primordial cells in a row. b. Isolated cell, uncon- 



VeSSCls, the gland assumes nected with the row, and undergoing development in its 



, . .. , original cell shape, c. Primary tube, formed by the fusion 



its Ultimate Shape and Clia- of the cells, d. Second stage of development of the thy- 



mus, showing bulgings of tube in different stages, which 

 Cavity ultimately become themselves divided. After Simon. 



racter. The closed 



VOL. II. 



M M 



