1871. 



NEW ENGLAND FARMER. 



647 



which is to protect the heart against friction 

 by contact with other organs. 



Tiie heart is a large, strong, hollow muscle, 

 the walls of which are composed of fibres run- 

 ning, some longitudinally, but most of them 

 in a spiral direction. It is divided in the di- 

 rection of its length, into two halves, each 

 representnig a single heart, divided into an 

 auricle and a ventricle ; thus we have a right 

 and a left side of the heart, — a right auricle 

 and a right ventricle ; a left auricle and a left 

 ventricle. The ventricles make up the body 

 of the heart, the auricles being, in reality, only 

 appendages to the heart proper, which serve 

 as reservoirs for the blood. Butchers call 

 them deafears. 



The walls of the left side of the heart are 

 thicker and stronger tlian those of the right, 

 and the reason is — the right side sends its con- 

 tents only to the lungs — a short distance — while 

 the left side projiels its blood to all parts of 

 the system. The walls of the auricles are 

 also much thinner than those of the ventricles. 



The auricles are separated from the ven- 

 tricles by triangular folds of membrane, which 

 perform the office of valves. Those which 

 separate the right auricle and ventricle are 

 called tricuspid valves, because they have 

 three points, and those which separate the 

 left auricle and ventricle are called bicuspid 

 or mitral valves, because they have two 

 points. There are also three valves at the 

 entrance of the aorta, the great artery of the 

 body, and the same number at the mouth of 

 the pulmonary artery. These six valves are 

 called semi-lunar, or half-moon, on account 

 of their peculiar shape. These several valves 

 are kept in position by small white cords, 

 called chordce tendince. The ventricles are 

 separated, one from the other, by a strong 

 muscular septum or partition ; and within 

 each ventricle are seen numerous fleshy col- 

 ums, called columnoi carnece. These are sup- 

 posed to aid in the contraction of the ven- 

 tricles. 



The channels of communication between the 

 heart and the se- eral parts of the body, and 

 through which the blood is propelled, are 

 the arteries, veins, and capillaries. 



The arteries are cylindrical tubes, composed 

 of three coats. The external or cellular 

 coat is composed of condensed cellular mem- 

 brane, and forms a strong, tough investment 

 to the artery, enabling it to resist the action 

 of the heart. The middle or Jihrous coat, by 

 its elasticity equalizes t ;e How of blood through 

 the vessels, and by its contractility enables 

 the arteries to close their divided extremities 

 when they are cut or torn, so as to prevent a 

 great loss of blood, unless the vessel be of 

 considerable size. Thw internal or serous coat 

 is a thin smooth membrane, which permits the 

 blood to flow with the least possible friction. 

 The arteries gradually diminish in size to- 

 wards their extremities, and finally terminate 

 in minute, hair-like vessels, called capillaries, 



which are too small to be seen with the naked 

 eye. 



The veins, like the arteries, are composed 

 of three coats, but they are much thinner, so 

 that the veins do not, like the arteries, retain 

 a cylindrical form when emptied of their con- 

 tents, but collapse, or become flattened. The 

 veins commence by minute vessels, called 

 radicals, which have their origin in the ca- 

 pilliary arteries. They unite, one with an- 

 other, to form larger and still larger branches, 

 until they terminate in two large trunks — the 

 ascending venacava, and the descending vena- 

 cava — which convey the blood directly to the 

 heart. 



The capillaries — so called on account of 

 their resemblance to hairs — are so minutely 

 distributed to every part of the body, as to 

 render it impossible to puncture the skin any- 

 where, without wounding some of them ; and so 

 small are they, in the human subject, that a 

 single one of them will not measure more than 

 l-o()00th of an inch in diameter. In the ca- 

 pillaries, the blood is brought into immediate 

 contact with all parts of all the tissues of the 

 body, and in them it parts with its nutritive 

 elements, so that these little microscopic ves- 

 sels constitute the medium through which the 

 functions of nutrition and secretion are per- 

 formed, as well as the channels of communi- 

 cation between the arteries and the veins. 



Having described the organs by which the 

 circulation of the blood is performed, we will 

 now trace its course through them ; and in 

 doing so, we begin at the left side of the 

 heart. 



By the contraction of the left ventricle, the 

 blood is forced through the semi-lunar valves 

 into the aorta or great arterial trunk, and 

 along its successive branches — the smaller ar- 

 teries — into the net- work of the capillaries. 

 Here the blood parts with its nutritive ele- 

 ments, and is changed from a scarlet to a 

 purple color, and is collected into the small 

 veins. It flows through their converging 

 branches into the vence cava, and finally into 

 the right auricle, which empties it through the 

 tricuspid valves into the 7-ic/ht ventricle. 



From the right ventricle, the blood is im- 

 pelled through the semi-lunar valves into the 

 jndmonary artery. This artery divides near 

 the heart, one branch leading to the right 

 lung, and the other to the left, so that through 

 this artery and its branches, the blood is car- 

 ried to the capillaries of the lungs, where it is 

 exposed to the action of the air. From the 

 capillaries of the lungs, the blood enters, in 

 converging streams, the pidmonary veins, 

 which return it to the heart and empty it into 

 the left auricle, which, in its turn, empties it' 

 through the bicusjnd or mitral valves into the 

 left ventricle, — the point whence we started. 



The course of the blood from the left side 

 of the heart through the arteries, capillaries, 

 and veins, back to the right side of the heart, 

 is called the greater or systemic circulation ; 



