644 THE POPULAR SCIENCE MONTHLY. 



the control of men's consciences icithout^ and beyond, the scope of 

 human legislation ; so that state education is a legitimate subject of 

 state control, while the support of a national Church is altogether be- 

 yond the sphere of national authority. 



THE BLOOD AND ITS CmCULATION.* 



By HERMAN L. FAIRCHILD. 



IN vertebrates alone is there a closed circulation a complete system 

 of tubes from whence the blood never escapes into the body-cavity. 

 We find an approach to it in the higher mollusks. Indeed, in power 

 and general efficiency, the circulation of the highest mollusks is great- 

 ly superior to that of the low vertebrates. Nevertheless, the perfectly 

 closed circulatory system of even the lowest vertebrates is of higher 

 type. Although the circulating system of the vertebrates is perfected 

 in principle, it still admits of very great and curious modifications. 



There exist in vertebrates three sets of capillary blood-vessels, 

 which are usually spoken of as three systems, although together they 

 constitute but a single circuit. They are distinguished as the body or 

 systemic circulation, the respiratory or pulmonary circulation, and the 

 liver or portal circulation. Connected with the blood-system by the 

 thoracic duct is the lymphatic circulation. 



The lymphatic system, which has previously been mentioned as the 

 second source of blood material, deserves some notice on account 

 of its intimate relation with the blood system of the vertebrates. 

 The lymphatics are minute capillary vessels, found in all parts of 

 the body of vertebrates, excepting, perhaps, the bulb of the eye, the 

 cartilages, and the bones. They unite to form, with the lacteals, the 

 thoracic duct, which was described in the article on digestion, in the 

 September number of the " Monthly." 



The office of the lymphatics is to collect the waste matter of the 

 tissues and return it to the blood, to be again used elsewhere, or, if 

 wholly useless, to be excreted from the body. They also collect the 

 blood which may be poured upon the tissues in excess of their needs. 

 The fluid which the lymphatics carry is called lymph. It is color- 

 less, and contains corpuscles resembling the white corpuscles of the 

 blood. 



The lacteals, which take the new food from the intestines, are 

 lymphatics modified for a special purpose, and, when they are not busy 

 with the chyle, they also carry lymph. 



The lymphatic tubes are provided with valves to keep the lymph 

 flowing toward the larger trunks. 



" Concliulcd from page 468. 



