224 Henry Fox 



the thymus. That a decided tension of the kind indicated is actually 

 produced is evidenced by the later behavior of the nerve — a subject 

 which I shall consider more fully when considering the modifications 

 of the fundus prgecervicalis. 



In a cat of 10.7 mm. the thymus has approximately the same char- 

 acteristics as in the pig just considered. It shows, however, no clear 

 lumen in any part. 



In a pig of 20 mm. (No. 543, Harvard collection, Figs. 23-25) the 

 thymus (Thy.) is considerably longer than hitherto and its ventral 

 extremity is slightly lobed. Its lumen has entirely disappeared, owing 

 to the thickening of its walls. The whole organ is thus composed of 

 small-celled epithelioid tissue, which in every respect l)ears a close resem- 

 blance to ordinary lymphoid tissue. The dorsal plate is closely wedged 

 in between the carotid gland and the fundus prgecervicalis (Fig. 23). 

 The th3'mus on the left side has completely separated from the pharynx, 

 while that on the right is still connected with it by an extremely thin 

 cord (Fig. 28). 



The separation from the pharynx is probably connected with the 

 inward flexure of the sinus piriformis and the consequent decrease in 

 the lateral diameter of the pharynx. The dorsal extremity of the 

 thymus and the attached carotid gland, however, retain their original 

 position, with the result that the connecting cord is drawn out to an 

 exceedingly thin strand, which subsequently constricts. 



In the cat of 15 mm. (Fig. 63) the thymus shows no special features. 

 It closely resembles the same structure in the pig just considered, but 

 is located at a relatively lower level. In the pig its dorsal extremity is 

 opposite the sinus piriformis, while in the cat it lies some distance 

 below it. 



In the 24 mm. pig (No. 64, Harvard collection. Figs. 29-31) the 

 thymus on each side is completely separated from the pharynx. Its 

 terminal thoracic portion has grown considerably l^elow the level of 

 the thjrroid. It is more swollen than hitherto and its distal portion is 

 subdivided into a considerable number of convolutions (Fig. 31, Thy.). 



The intermediate cervical connective persists as an exceedingly thin 

 solid cord (Cv.C). 



The dorsal plate is so intimately fused with the carotid gland and 

 fundus prgecervicalis (F.Pc.V.) as to be distinguishable from them only 

 with difficulty. 



