STRUCTURE OF THE VEINS. 



143 



taken at three inches per minute, that through the pulmo- Motion of the 

 nary being five times as quick, the length of the capillary tube blood in the 

 to be passed -^ of an inch, so that the passage from the ar- ca P lllanes - 

 tery to the vein may be accomplished in less than one second. It is to 

 be remarked, however, that all parts of the cylindrical stream do not 

 move with equal rapidity. Those parts which are nearest to the wall of 

 the vessel are spoken of as the still layer, from their tardy movement. 

 It is in this that the white corpuscles may be seen. 



fig. 64. Fig. 64 shows a portion of a small 



vessel from a frog's foot: <z, <z, red blood 

 elliptic cells, occupying the axis of the 

 vessel, and exterior to them, moving 

 more slowly, or occupying the still lay- 

 er, the white spherical cells ; b, b, nucle- 

 ated epithelium. 



4th. The veins have a structure in 

 some respects different from The veins : 

 that of the arteries. Their their structure. 

 -, elastic coat is by no means so much de- 

 veloped, and their muscularity less dis- 

 tinct. With the exception of those of 

 the lungs, abdominal viscera, and brain, 

 their interior is furnished with valves of single, double, or triple flaps, in 

 all instances opening toward the heart. The blood flows equably in 

 them, the pulsating action of the ventricles having disappeared in the 

 capillaries. Since they present an aggregate capacity two or three times 

 that of the arteries, the motion of the circulation in them is proportion- 

 ally slower. Fig. 65 is a diagram showing the manner in which the 

 valves open when the blood flows in the course indicated by the arrows. 



Fig. 66. 



Fig. 65. 



Valves of veins open. 



White corpuscles in the still layer. 



Valves of veins shut. 



Fig. 66 shows their application to each other, or to the sides of the vein, 

 and the consequent bulging of that vessel when the current, as indicated 

 by the arrows, is in the opposite direction. 



Having now described the structure and action of the heart, the arte- 

 ries, capillaries, and veins respectively, as far as is necessary, it remains 

 to group those actions together, and present the theory of the circulation 

 at one view. 



But, before entering on this, it is proper to offer an ar- Error of the doc- 

 gument against the doctrine of those physiologists who still h eart is t h e sole 

 maintain that the circulation is wholly dependent on the heart, cause of the cir - 



. . T . culation. 



and fhnt that orcran is entirely competent to carry it on. 



