THE CIRCULATION 177 



artery, which blood, after it has passed through the capillaries, 

 mingles with the blood of the portal capillaries, and is by them 

 returned to the heart through the hepatic vein. 



49. Assimilation. The crowning act of the circulation 

 the furnishing of supplies to the different parts of the body 

 is effected by means of the capillaries. The organs have been 

 wasted by use ; the blood has been enriched by the products 

 of digestion. Here, within the meshes of the capillary net- 

 work, the needy tissues and the needed nutriment are brought 

 together. By some mysterious chemistry, each tissue selects 

 and withdraws from the blood the materials it requires, and 

 converts them into a substance like itself. This conversion 

 of lifeless food into living tissue is called assimilation. The 

 process probably takes place at all times, but the period espe- 

 cially favorable for it is during sleep. Then the circulation is 

 slower and more regular, and most of the functions are at 

 rest. The body is then like some trusty ship, which, after a 

 long vogage, is " hauled up for repairs." {Read Note 14.) 



50. Lymph and the Lymphatic System. As the blood circu- 

 lates through the capillaries of the body, a good deal of its 

 liquid portion, the plasma, passes through the walls of the 

 capillaries into the surrounding tissue cells. This plasma is 



14. Assimilation in Repair. " Most animals have the power to 

 repair, to a greater or less extent, the mutilations they undergo. In 

 man, if the skin is torn off, a new skin heals over the injury, and a broken 

 bone is caused to re-unite by the deposit of bone tissue between the frag- 

 ments. But among the lower animals this power is carried to a high 

 degree. The tail of a lizard, if cut off, will quickly form anew, although 

 of a complex structure ; and spiders and crabs are able to develop new 

 claws upon the stumps of broken ones. Observations made on salaman- 

 ders, or water-lizards, show the still more remarkable fact that the eye 

 and a part of the head may be entirely restored. Certain kinds of earth- 

 worms can reproduce a large portion of their bodies, and any fragment of 

 the hydra is able to restore itself, and become a complete creature after its 

 kind. Assimilation is especially active in early years, while the body is 

 growing; for this reason, among others, the perfect health of children 

 requires that they shall give a greater number of hours to sleep deep, 

 regular, and undisturbed sleep than is needed in later life." Milne 

 Edwards. , 



49. What is stated of the injuries to the blood-vessels ? 

 60. How does alcohol affect the heart's action ? 



