LEPUS CUNICULUS 319 



atrio-ventricular valves. Following the sharp systole the atria 

 pass through a relatively long period of slow distension or diastole. 

 This period can only be considered long when compared with the 

 time occupied by systole ; actually, of course, the whole time 

 occupied by the two is measured in fractions of a second. In a 

 similar manner the ventricle undergoes a sharp systole followed by a 

 longer diastole, and naturally the end of ventricular diastole coin- 

 cides with atrial systole. The atrial diastole, however, is not 

 similarly related to ventricular systole, and there is a period of 

 time when all chambers of the heart are in diastole. The con- 

 traction of the ventricles expels the blood through the aorta and 

 pulmonary artery. These vessels having elastic walls enlarge 

 with the inflow of blood, and as soon as the ventricular con- 

 traction relax, their elasticity tends to drive the blood back into 

 the ventricle, a proceeding that is stopped by the rapid closure 

 of the semilunar valves. It will be recalled that behind two 

 of the aortic valves lie the apertures of the coronary arteries, 

 and the blood is driven into the coronary circulation by this same 

 reverse wave of pressure that shuts the valves. The elasticity of 

 the walls of the two main arteries is sufficient to drive the blood 

 onwards to the body or lungs as the case may be. 



In listening to the heart beating within the thorax two distinct 

 sounds can readily be made out. The first is a low, dull sound, and 

 the second a higher, sharper one. The latter is found to be due to 

 the closure of the semilunar valves at the bases of the two arterial 

 trunks, while the former is the result of several causes, the principal 

 among which are the tensing of the mass of the ventricular muscles, 

 marking the beginning of systole and the practically simultaneous 

 closure of the atrio-ventricular valves. It should be obvious then 

 that imperfect structure or functioning of either sets of valves will 

 affect the quality of the corresponding heart sound, and so can be 

 detected by listening to the beat and comparing its sound with those 

 made by the normal heart. 



Urogenital System. 



As in the frog we find that the excretory and reproductive 

 systems are closely related anatomically, and while the sperms do 

 not actually utilise the kidney ducts, the ends of the reproductive 

 and excretory ducts are closely related in the two sexes, and the 

 same external aperture serves for the emission of the urine and 

 the genital products. This external opening is independent of the 

 opening of the alimentary canal, and as it is concerned with the 

 reproductive functions it naturally differs considerably in the two 

 sexes. 



