290 ESSENTIALS OF ZOOLOGY 



left anterior vena cava just below the subclavian vein. The 

 whole system is interrupted by numerous lymphatic glands, 

 and is well supplied with valves directing the flow onwards 

 to the thoracic duct, which is also supplied with valves. The 

 lymph of the left side of the head is received by the thoracic 

 duct, but that of the right side is discharged into the corre- 

 sponding vein of that side. 



Endocrine Organs. The blood has an important inte- 

 grating function besides being the means of carrying food, 

 oxygen, and waste products. It is made to convey products 

 which intimate rapidly changes in conditions. The changes 

 undergone by the blood as it circulates are conveyed generally 

 by chemical messages. In addition to messages from one organ 

 to another such as those conveyed take place along the alimen- 

 tary canal during the digestion of the food, there are special 

 organs which intimate by means of the blood, into which they 

 send small quantities of their internal secretions, hormones, or 

 endocrines, the need for action or a change in growth. 



The thymus gland is derived in the embryo from ventral 

 buds of the branchial clefts, mainly from the third. They 

 form in the young rabbit an organ of great size which occupies 

 the mediastinum. It gradually atrophies with age, and is 

 plainly associated with the growing years and in restraining 

 the development of the gonads. 



The thyroid, which is developed from the floor of the 

 pharynx, and the history of which has already been explained, 

 yields a proteid body united with iodine, and this secretion 

 has been found to have an important relationship with growth 

 and in promoting the development of sex. If too active it 

 gives rise to exophthalmic goitre ; and if atrophied, to the 

 disease myxoedema, cretinism and failure to develop the organs 

 of sex. 



The parathyroids are developed from branchials 3 and 4 

 as dorsal outgrowths, and are usually so closely associated 

 with the thymus as to be difficultly isolated therefrom. They 

 appear to act as neutralisers of the action of the thymus. 



The pituitary body, derived from the pituitary outgrowth 

 of the ectoderm of the stomodeum and attached to the 

 infundibulum of the brain, has also important secretions, 



