ANATOMY OF THE HEART. 



35 



Physiological Anatomy of the Heart. 



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

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

 space, three inches to the left of the median line or one inch within the line of 

 the left nipple. Its weight is 

 from eight to ten ounces in 

 the female, and from ten to 

 twelve ounces 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, or may be said 

 to be attached by the great 

 vessels to the posterior wall 

 of the thorax, while the apex 

 is free and capable of a cer- 

 tain degree of motion. The 

 whole organ is enveloped in a 

 fibrous sac called the pericar- 

 dium. This sac is lined by a 

 serous membrane, which is at- 

 tached to the great vessels at 

 the base and reflected over its 

 surface. The membrane is lu- 

 bricated by a drachm or two FIG. 11. Heart in situ. (Dalton, in Flint, "on the Heart.") 



Of fluid, SO that the movements a ' & ' c ;^ tc " [ ibs 5 a < 2 - 3, etc intercostal spaces; vertical line, median line; 



triangle, superficial cardiac region ; x on the fourth nb, nipple. 



of the heart are normally ac- 

 complished without any friction. The serous pericardium does not present any dif- 

 ferences 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 cava3 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 descendens, 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 cavity are quite thin as 

 compared with the ventricles, measuring about one line. They are composed of mus- 

 cular fibres arranged in two layers, one of which, the external, is common to both auri- 

 cles, and the other, the internal, is proper to each. These muscular fibres, although 

 involuntary in their action, belong to the striated, or what is termed voluntary variety, 

 and are similar in structure to the fibres of the ventricles. The fibres of the auricles are 

 much fewer than those of the ventricles. Some of them are looped, arising from a 

 cartilaginous ring which separates the auricles and ventricles, and passing over the 

 auricles ; and others are circular, surrounding the auricular appendages and the open- 

 ings of the veins, extending, also, a short distance along the course of these vessels. 

 One or two valvular folds are found at the orifice of the coronary vein, preventing a 

 reflux of blood, but there are no valves at the orifices of the venae cavro. 



The left auricle receives the blood which comes from the lungs by the pulmonary 



