ACTION OF THE HEART. 331 



Kolliker says, they "appear to become transformed into the 

 larger vessels by the super-addition of layers, both on their 

 inner and onter sides; their own coat, meanwhile, coalescing 

 with these, and being, perhaps, continued into the fibrous 

 layer of the tunica interna." 



The veins differ from arteries in having less elastic mate- 

 rial in their coats. They are tougher, and in some parts are 

 provided with an abundant muscular coat. Veins are pro- 

 vided with valves, which are very numerous, where gravita- 

 tion is opposed to the blood's flow, as in the veins of the 

 limbs. The valves are composed of folds from the middle 

 and inner venous tunics. 



ACTION OF THE HEAET. 



The action of the heart is progressive, and consists first in 

 the contraction of the auricles for the propulsion of blood 

 into the ventricles. The auricles dilate, and the ventricles 

 contract; there is, then, a pause in the heart's action. At 

 the time that the ventricles contract, the impulse of the heart 

 is felt against the left side. This is due to a complete move- 

 ment of the heart, and a rolling from right to left, in accord- 

 ance with the arrangement of the muscular fibres, though the 

 ventricles also elongate, as shown by "William Harvey. In 

 describing the action of the heart, this illustrious observer 

 says: " First of all the auricle contracts, and in the course 

 of its contraction throws the blood (which it contains in 

 ample quantity as the head of the veins, the storehouse and 

 cistern of the blood) into the ventricle, which being filled, the 

 heart raises itself straightway, makes all its fibres tense, con- 

 tracts the ventricles, and performs a beat, by which beat it 

 immediately sends the blood supplied to it by the auricle, 

 into the arteries ; the right ventricle sending its charge into 

 the lungs by the vessel which is called vena arteriosa, but 



