70 ESSENTIALS OF 2O0L0GY 



important duty, as possibly also is the formation in its tissues of 

 lymphocytes, one type of white blood corpuscle. In mammals the 

 spleen is also believed to accumulate iron freed by the metabolism 

 of other tissues. This iron is subsequently used in the formation of 

 hemoglobin. 



The two thyroid glands are small and lie in front of the glottis 

 under the floor of the mouth. There is one on each side of the hyoid 

 apparatus. The secretion and functions are discussed in the chapter 

 on Endocrine Glands and Their Functions. 



A thymus gland lies under the skin behind the tympanic membrane 

 on each side. It is partly covered with muscle and is small. Further 

 discussion of it will be taken up in the chapter on Endocrine Glands 

 and Their Fu7ictions. 



* 



Circulatory System 



The circulatory system comprises the blood vascular system and the 

 lymphatic system. The two systems are closely interrelated in that 

 they both carry to the tissues of the body nutritive material neces- 

 sary for metabolism and remove from them to the excretory organs, 

 waste products of body activity. They differ in several respects; 

 the lymph neither contains red blood corpuscles for transporting 

 oxygen nor moves in a continuous closed vascular circuit as does 

 the blood. Other differences will be noted in the discussion. 



The Blood Vascular System. — The blood moves through a closed 

 system of tubelike vessels of various sizes which distribute it to all 

 parts of the body. The pump is the heart, which, by its contractions, 

 forces the blood to flow to the tissues. 



The blood vessels leading away from the heart are the arteries. 

 When these reach the tissues, they break up into very small vessels, 

 the capillaries. The vessels leading back to the heart are the veins. 

 The arteries and veins are connected by the capillaries. 



Blood is comprised of a clear liquid called the plasma, suspended 

 in which are blood corpuscles of three kinds, the red blood corpuscles 

 or erythrocytes, the white blood corpuscles or leucocytes, and the 

 spindle cells or thrombocytes. In addition, the blood may contain 

 dissolved nutritive substances from the digestive system, waste 

 products from tissue repair and destruction, hormones being trans- 

 ported from organs of one part of the body to another, or foreign 

 substances accidentally introduced. 



