Courtesy of Johns Hopkins Bulletin and Dr. Eben C. Hill 



BLOOD VESSELS REACH ALL PARTS OF THE BODY 



If we could see the arteries and veins in any living animal, with the connecting capil- 

 laries, the entire mass would practically correspond to the entire body. An X-ray 

 picture of a baby's arm, showing the arteries 



The Red Corpuscles The color of the blood is due to a yellowish pig- 

 ment called hemoglobin. This readily combines with oxygen and gives it 

 up again, according to the chemical conditions to which it is exposed. For 

 this reason the red corpuscles play an important role in breathing (see 

 page 205). 



Red blood cells originate by cell-division from special cells in the red 

 marrow of bones, which occurs in the ribs, the vertebrae, and in the upper 

 ends of the armbone and thighbone. In the embryo, red corpuscles originate 

 in the liver and in the yellow marrow of the long bones. Each corpuscle 

 starts out with a nucleus. But among the mammals this soon disappears. 

 The older corpuscles in the mammals go to pieces, and their hemoglobin is 

 taken up by the liver and converted into part of the bile (see page 168). 



The largest red corpuscles are found among the amphibians. Even with 

 the low power of a microscope we may easily see the elliptical disks in the 

 flowing blood of a frog's web or a tadpole's tail. 



How Is the Blood Circulated? 



The Heart and the Vessels^ The blood is kept moving by the rhythmic 

 contractions of the pumping organ, the heart. Blood comes into the heart 

 through vessels which are called veins; blood flows out of the heart in tubes 

 known as arteries. The arteries branch and divide again and again, reach- 



iSee Nos. 1, 2 and 3, pp. 198-199. 

 189 



