32 



CIRCULATION OF THE BLOOD. 



PHYSIOLOGICAL AXATOMY OF THE HEART. 



The heart of the human subject is a pear-shaped, muscular organ, situ- 

 ated in the thoracic cavity, with its base in the median line and its apex at 

 the fifth intercostal space, three inches (7'6 centimetres) to the left of the 

 median line, or one inch (2-5 centimetres) within the line of the left nipple. 



Its weight is eight to 

 ten ounces (227 to 283 

 grammes) in the female, 

 and ten to twelve ounces 

 (283 to 340 grammes) 

 in the male. It has 

 four distinct cavities ; a 

 right and a left auricle, 

 and a right and a left 

 ventricle. Of these, 

 the ventricles are the 

 more capacious. The 

 heart is held in place 

 by the attachment of 

 the great vessels to the 

 posterior wall of the 

 thorax; while the apex 

 is free and capable of 

 a certain degree of mo- 

 on the heart"). tion. The whole organ 



FIG. 11. Heart in situ (Dalton, in Flint, 

 a, b. c etc., ribs ; 1, 2, 3 etc., intercostal spaces 



line ; triangle, superficial cardiac region ; x on the fourth rib, 

 nipple. 



a is enveloped in a fibrous 



sac called the pericar- 

 dium. This sac is lined by a serous membrane, which is attached to the 

 great vessels at the base and reflected over its surface. The membrane is 

 lubricated by about a drachm (3-7 c. c.) of fluid, so that the movements of 

 the heart are normally accomplished without any friction. The serous peri- 

 cardium does not present any differences from serous membranes in other 

 situations. The cavities of the heart are lined by a smooth membrane called 

 the endocardium, which is continuous with the lining membrane of the 

 blood-vessels. 



The right auricle receives the blood from the venae cavae and empties it 

 into the right ventricle. The auricle presents a principal cavity, or sinus, as 

 it is called, with a little appendix, called, from its resemblance to the ear of 

 a dog, the auricular appendix. It has two large openings for the vena cava 

 ascendens and the vena cava descendeus respectively, with a small opening 

 for the coronary vein which brings the blood from the substance of the heart 

 itself. It has, also, another large opening, called the auriculo- ventricular 

 opening, by which the blood flows into the ventricle. The walls of this cav- 

 ity are quite thin as compared with the ventricles, measuring about one line 

 (2-1 mm.). They are composed of muscular fibres arranged in two layers, 



