THE MAIN FACTS OF THE CIRCULATION, 137 



considerable force and marked intermittence, according to the amount of 

 collateral communication. 



When a corresponding vein is severed, the flow of blood, which is chiefly 

 from the distal cut end, that in connection with the capillaries, is not jerked 

 but continuous ; the blood comes out with comparatively little force, and 

 " wells up " rather than " spurts out." The flow from the'proximal cut end, 

 that on the heart side, may amount to nothing at all or may be slight or 

 may be considerable, depending on the presence or absence of valves and the 

 amount of collateral communication. 



When an artery is ligatured the vessel swells on the proximal side, toward 

 the heart, and the throbbing of the pulse may be felt right up to the liga- 

 ture. On the distal side the vessel is empty and shrunk, and no pulse can 

 be felt in it unless there be free collateral communication. 



When a vein is ligatured the vessel swells on the distal side, away from 

 the heart, but no pulse is felt ; while on the proximal side, toward the heart, 

 it is empty and collapsed unless there be too free collateral communication. 



103. When the interior of an artery for instance, the carotid is 

 placed in communication with a long glass tube of not too great a bore, held 

 vertically, the blood, immediately upon the communication being effected, 

 may be seen to rush into and to fill the tube for a certain distance, forming 

 in it a column of blood of a certain height. The column rises not steadily, 

 but by leaps, each leap corresponding to a heart-beat, and each leap being 

 less than its predecessor ; and this goes on, the increase in the height of the 

 column at each heart-beat each time diminishing, until at last the column 

 ceases to rise and remains for a while at a mean level, above and below 

 which it oscillates with slight excursions at each heart-beat. 



To introduce such a tube an artery say the carotid of a rabbit is laid bare, 

 ligatured at a convenient spot, V Fig. 37, and further temporarily closed a little 

 distance lower down nearer the heart by a small pair of u bull-dog" forceps, bd, 

 or by a ligature which can be easily slipped. A longitudinal incision is now made 

 in the artery between the forceps, bd, and the ligature /' (only the drop or two of 

 blood which happens to remain inclosed between the two being lost) ; the end of 

 the tube, represented by c in the figure, is introduced into the artery and secured 

 by the ligature /. The interior of the tube is now in free communication with the 

 interior of the artery, but the latter is by means of the forceps at present shut off 

 from the heart. On removing the forceps a direct communication is at once estab- 

 lished between the tube and the artery below ; in consequence the blood from the 

 heart flows through the artery into the tube. 



This experiment shows that the blood as it is flowing into the carotid is 

 exerting a considerable pressure on the walls of the artery. At the moment 

 when the forceps are removed there is nothing but the ordinary pressure of 

 the atmosphere to counterbalance this pressure within the artery, and con- 

 sequently a quantity of blood is pressed out into the tube ; and this goes on 

 until the column of blood in the tube reaches such a height that its weight is 

 equal to the pressure within the artery, whereupon no more blood escapes. 

 The whole column continues to be raised a little at each heart-beat, but sinks 

 as much during the interval between each two beats, and thus oscillates, as 

 we have said, above and below a mean level. In a rabbit this column of 

 blood will generally have the height of about 90 cm. (3 feet) ; that is to say, 

 the pressure which the blood exerts on the walls of the carotid of a rabbit is 

 equal to the pressure exerted by a column of rabbit's blood 90 cm. high. 

 This is equal to the pressure of a column of water about 95 cm. high, and to 

 the pressure of a column of mercury about 70 mm. high. 



If a like tube be similarly introduced into a corresponding vein say 

 the jugular vein it will be found that the column of blood, similarly formed 



