THE MOTION OF THE BLOOD. 193 



The heart of mammalia and birds has no peculiarity necessary to be mentioned 

 here. In most amphibious animals, the arteries of the system as well as of the 

 lungs spring from the right ventricle, with which the left, that sends off no vessel, 

 communicates: hence their circulation continues under water. In amphibious 

 mammalia and diving birds, some vessels, especially one vena cava, are dilated, to 

 form a receptacle during the suspension of respiration. The heart of fish is ex- 

 tremely small, and has but one auricle and ventricle, the latter propelling the 

 blood to the gills, from which it streams to the system through a large artery. 

 Neither blood-vessels nor absorbents have been discovered in insects, yet a large 

 tube pulsates in their back ; and Professor Carus has lately discovered a circu- 

 lation in them through a granular substance, the streams running to the posterior 

 end of this vessel, and issuing again from its anterior end. With respect to the 

 mottusca, the cuttle-fish has three detached hearts, consisting of a ventricle only, 

 two for the gills and one for the aorta ; the rest have a simple heart, the blood 

 of the cava passing through the gills before it reaches the heart. The same 

 is the case with the crustacea, and their heart has no auricle. Worms have 

 circulating vessels distinctly contracting and dilating, but no heart, and their 

 veins communicate with the general cavity of the body, and probably absorb. 

 Zoophytes have no heart, nor circulating system, properly so called. In the 

 echinus, indeed, there are two vessels that run along the intestines, and are 

 thought to be an aorta and vena cava. But currents may occur, and not be 

 perceptible if the fluid is colourless, or has no globules ; and currents have been 

 lately discovered by Mr. Lister in some zoophytes exactly similar to the currents 

 long observed under the microscope in the tubes of stone- wort ; the streams run- 

 ning first in one direction, on the internal surface of a tube, and then returning 

 in another on the same surface. Such streams on surfaces or through cells are 

 very wonderful. 



According to Dr. M. Hall, when the office of a part in brutes is simple, the 

 distribution of blood-vessels is simple, as in the fin or tail of a fish, and the arteries 

 chiefly become veins : but when its office is complicated, as in the toes of the 

 frog, or the blood has to be thoroughly exposed to air, as in the lungs, the 

 arteries give off a number of branches, which do not diminish in diameter or 

 give off others, and are peculiarly called by Dr. Hall capillaries, as large as, or 

 larger than, their parent branch, freely anastomosing, not diminishing in size, 

 nor giving origin to or running into the sides of veins. In the lungs, the 

 large vessels presently split into capillaries; in the systemic arteries, the vessels 

 diminish and subdivide considerably beforehand. 



Vegetables have no central organ of circulation. The sap rises ordinarily 

 through the cells, or, according to Decandolle, the intercellular spaces of the 

 wood. Some plants are altogether cellular. The vessels in the wood of those 

 which are vascular are found to contain air only, and the sap sometimes takes 

 so circuitous a route, is so diffused, and so subsides to the lowest parts, that it 

 cannot, in all cases at least, be confined to vessels. The sap rises chiefly in the 

 newest layer of wood, called alburnum. But when the buds are preparing for 

 developement, and the leaves are not yet complete, the sap is termed nursling sap, 

 and ascends through the oldest and innermost layer of wood, and passes through 

 unknown channels to the buds, combining probably with nutriment formerly 



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