186 BLOOD-CURRENT IN THE SMALLEST VESSELS. 



250 mm. Hg. = 3'21 metres of blood.] The work of the heart at each 

 systole is O188 X 3*21 0'604 kilogramme-metres. If the number of 

 beats =75 per minute, then the work of the left ventricle in 24 hours 

 = (0-604 X 75 X GO x 24) = 65,230 kilogramme-metres. While the 

 " work" done by the right ventricle is about one-third that of the left, and 

 therefore =2 1,740 kilogramme-metres. Both ventricles do work equal 

 to 86,970 kilogramme-metres. A workman during eight hours produces 

 300,000 kilogramme-metres i.e., about four times as much as the heart. 

 As the whole of the work of the heart is consumed in overcoming the 

 resistance within the circulation, or rather is converted into heat, the 

 body must be partly warmed thereby (425'5 gramme-meters are equal 

 to 1 heat-unit i.e., the force required to raise 425'5 grammes 

 to the height of 1 metre may be made to raise the temperature of 

 1 cubic centimetre of water 1C.) So that 204,000 "heat-units" are 

 obtained from the transformation of the kinetic energy of the heart. 



One gramme of coal when burned yields 8,080 heat-units, so that 

 the heart yields as much energy for heating the body as if about 

 25 grammes of coal were burned within it to produce heat. 



94. Blood-Current in the Smallest Vessels. 



Methods. The most important observations for this purpose are 

 made by means of the microscope on transparent parts of living animals. 

 Malpighi was the first to observe the circulation in this way in the 

 lung of a frog (1661). 



The following parts have been employed : the tails of tadpoles and small fishes ; 

 the web, tongue, mesentery, and lungs of curarised frogs (Cowper, 1704) ; the wing 

 of the bat, the third eyelid of the pigeon or fowl, the mesentery ; the vessels of the 

 liver of frogs and newts (Gruithuisen), of the pia mater of rabbits, of the skin on 

 the belly of the frog, of the mucous membrane of the inner surface of the human 

 lip (Hiiter's Cheiloangioscope, 1879) ; of the conjunctiva of the eyeball and eyelids. 

 All these may be examined by reflected light. 



[EntOptical appearances of the circulation (Purkinje, 1825). Under certain 

 conditions a person may detect the movement of the blood-corpuscles within the 

 blood-vessels of his own eye. The best method is that of Rood, viz., to look at the 

 sky through a dark blue glass, or through several pieces of cobalt glass placed over 

 each other (Helmholtz)]. 



Form and Arrangement Of Capillaries. Regarding the form and arrange- 

 ment of the capillaries, we find that 



1. The diameter, which in the finest, permits only the passage of single corpuscles 

 in a row one behind the other may vary from 5 /* - 2 /u, so that two or more 

 corpuscles may move abreast when the capillary is at its widest. 



2. The length is about 0'5 mm. They terminate in small veins. 



3. The number is very variable, and the capillaries are most numerous in those 

 tissues, where the metabolism is most active, as in lungs, liver, muscles less 

 numerous in the sclerotic and in the nerve-trunks. 



