BLOOD-CURRENT IN THE SMALLER VESSELS. 1 37 



much as the heart. As the whole of the work of the heart is consumed in over- 

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

 the body must be partly warmed thereby (425 '5 gramme-metres 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 1 C). 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 8080 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 SMALLER 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 cir- 

 culation 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 ; the wing of the bat ; the third eyelid of the 

 pigeon or fowl ; the mesentery ; the vessels of the liver of frogs and newts, pia mater of rabbits, 

 the skin on the belly of the frog, the mucous membrane of the inner surface of the human lip 

 {Hater's Cheilangioscope, 1879); the conjunctiva of the eyeball and eyelids. All these may be 

 examined by reflected light. 



[Holmgren's Method. In studying the circulation in the frog's lung, it must be inflated. 

 A cannula with a bulge on its free end is placed in the larynx, while to the other end is fixed a 

 piece of caoutchouc tubing. The lung is inflated and then the caoutchouc tube is closed, after 

 which the lung is placed in a chamber with glass above and below, and examined microscopic- 

 ally.] 



[Entoptical appearances of the circulation (Purbinje, 1815). 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 arrangement of the capil- 

 laries, 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 /x to 2 /*, so that 2 or more corpuscles may move abreast 

 when the capillary is at its widest. 



2. The length is about "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. 



4. They form numerous anastomoses, and give rise to networks, whose form and arrangement 

 -are largely determined by the arrangement of the tissue elements themselves. They form 

 simple loops in the skin, and polygonal networks in the serous membranes, and on the surface 

 of many gland tubes ; they occur in the form of elongated networks, with short connecting 

 branches in muscle and nerve, as well as between the straight tubules of the kidney ; they con- 

 verge radially towards a central point in the lobules of the liver, and form arches in the free 

 margins of the iris, and on the limit of the sclerotic and cornea. 



[Direct Termination of Arteries in Veins. Arteries sometimes terminate directly in veins, 

 without the intervention of capillaries, e.g., in the ear of the rabbit, in the terminal phalanges 

 of the fingers and toes in man and some animals, in the cavernous tissue of the penis. They 

 may be regarded as secondary channels which protect the circulation of adjacent parts, and they 

 may also be related to the heat-regulating mechanisms of peripheral parts (Hoyer).~\ 



In connection with the termination of arteries in capillaries, it is important to ascertain if the 

 arterioles are terminal arteries, i.e., if they do not form any further anastomoses with other 

 similar arterioles, but terminate directly in capillaries, and thus only communicate by capillaries 

 with neighbouring arterioles or the arteries may anastomose with other arteries just before 

 they break up into capillaries. This distinction is important in connection with the nutrition 

 of parts supplied by such arteries {Cohnheim). 



Capillary Circulation. On observing the capillary circulation, we notice that 

 the red corpuscles move only in the axis of the current (axial current), while the 

 lateral transparent plasma-current flowing on each side of this central thread is 

 free from these corpuscles. [The axial current is the more rapid.] This plasma 

 layer or " Poiseuille's space " is seen in the smallest arteries and veins, where are 



