Page:Popular Science Monthly Volume 19.djvu/662

644 the control of men's consciences without 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 beyond the sphere of national authority.



N 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 greatly 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 colorless, 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. 