ANATOMY AXD HISTOLOGY OF THE THYMUS GLAND 363 



prelaryngeal and cervical lymph glands and into nodes lying dorsal to the 

 sternum. 



Innervation. A few nerves, coming from the cervical sympathetic 



(/ 



Fig. 2. Thymus gland of eight-day-old rabbit with included fragment of para- 

 thyroid (p), magnification 720. 



and vagus, penetrate the organ with the blood vessels. Twigs from the 

 phrenics and the descendens hypoglossi may enter the capsule. 



Embryology. 



Origin. The thymus develops from the endoderm of the third and, 

 occasionally, the fourth branchial pouches, as is indicated by the stippled 

 areas in Fig. 3. The portion, if any, formed from the fourth pouch 

 usually remains in association with the superior parathyroid glands. Ac- 

 cording to Grosser, the median portion of the third pouch narrows and 

 extends caudally, forming the ductus pharyngobranchialis III, which soon 

 degenerates completely, leaving the thymus anlage free. 



Development. The anlage is at first cylindrical in shape, but the 

 lumen is soon lost, through rapid cell proliferation, and the whole mass 

 migrates in a caudal direction, coming to rest in the cephalic part of the 

 thorax. If the migration is not complete, small portions of the thymus 

 often remain in association with the inferior parathyroids (glandulse para- 

 thyroidege III). This circumstance makes the results of thymectomy 

 difficult to interpret on account of the uncertainty as to whether all the 



