342 ZOOLOGY 



puscles are broken down, and probably white blood cells are 

 produced. The thyroid and thymus glands are paired glands 

 which are developed in the neck in relation to the embryonic 

 branchial arches. The function of the thymus is uncertain. 

 There is iodine in the thyroid secretion. In man the disease 

 or removal of the thyroid produces serious disturbances in 

 development. It may produce physical degeneracy and defor- 

 mity, and idiocy. 



The adrenal bodies are placed near the kidneys, as the name 

 indicates. They secrete into the blood a substance known as 

 adrenalin which among other things causes the muscles around 

 the blood vessels to continue in a state of contraction. In 

 this way the blood pressure is controlled. Removal produces 

 derangement of functions and death. 



The pituitary body is in part a ventral outgrowth from the 

 floor of the brain (see inf., Fig. 172, B). This fuses with tissues 

 arising from the pharynx. Excess or deficiency of the secretion 

 of this organ causes changes in the growth rate of certain organs 

 and in a number of the metabolic processes. Removal of the 

 organ in dogs causes functional disturbance and death within 

 a fortnight. 



358. In Figs. 1 66 to 171 will be found diagrams of the 

 circulation in the principal groups of vertebrates. It will be 

 seen that there is a progressive complication of the structure, 

 involving the heart, veins, and arteries, as we ascend the scale. 

 These changes accompany and are partly caused by the change 

 from gills to lungs. See also the accompanying table. Locate 

 the vessels and trace the changes indicated by means of larger 

 texts. 



359. In fishes the blood passes through the heart only once 

 in making the circuit of the body. In all the air-breathing 

 forms at least a part of it returns twice, passing from the 

 heart to the lung, then back to the heart, and thence to the 

 system and back to the heart again. In amphibia and reptiles 

 the blood from the lung and from the system mix somewhat 

 in the heart, because the partition between the right and left 

 sides is not complete, but in birds and mammals the two sides 



