248 PHYSIOLOGY AT THE FAEM. 



ventricle itself, tighten the membranous segments of the valve 

 and close the orifice. As soon as the contraction of the ven- 

 tricle is finished, it immediately expands, and the valve com- 

 manding the orifice leading from the auricle being relaxed, 

 the blood enters from that auricle to fill, without fully distend- 

 ing, the ventricle ; at the same time the blood is prevented 

 from regurgitating into the ventricle from the aorta by the 

 semilunar segments of the valve in the orifice of the aorta, 

 which are expanded so as to close the orifice under the pressure 

 of the column of blood in the direction of the heart, produced 

 by the slight contraction of the aorta due to the elasticity of 

 its coats, and consequent on a corresponding expansion propa- 

 gated along the aortic system when the blood is forced into it 

 by the contraction of the ventricle. That is to say, the portion 

 of blood thrown into the aorta by the contraction of the left 

 ventricle creates a wave in the blood of the aortic system with 

 a corresponding undulatory distention of the coats of that 

 system of vessels. But the elasticity of their coats produces 

 an immediate contraction, and this contraction of the aorta, 

 near its junction with the ventricle, would cause a regurgita- 

 tion of the blood into the ventricle at the moment of its expan- 

 sion, did not the semilunar segments expand under the pressure 

 of the returning column of blood, and intercept its progress. 

 The undulatory movement of the aortic system is the pulse, 

 which is felt when the finger is applied to an artery ; that is to 

 say, to any ramification of that system. The pulse in an artery 

 distant from the heart is in a slight degree later than in one 

 close to the heart. This difference is not, however, unequivo- 

 cally perceived, unless when the pulse is slower than usual. 

 As was stated above, the ventricle expands immediately after 

 its contraction, and becomes filled with blood without being 

 distended, under the influx of blood from the auricle through 

 the auriculo-ventricular orifice. Now with very little delay the 



