580 Anatomy, Physiology, and Pathology of the 



This valve is called the valvula tricuspis, being divided into three 

 portions, each of which tends from its base which is attached 

 to the sides of the ventricle towards a loose or floating apex. 

 From the pulmonary artery the blood enters the capillaries of 

 the lungs, where it undergoes a peculiar change, hereafter to be 

 explained, and is thence conducted back again to the heart by the 

 pulmonary veins. The action of the artery in assisting the on- 

 ward course of the blood would drive a portion of it into the 

 ventricle, but this is prevented by three valves of a semilunar 

 form which guard the mouth of the vessel. The pulmonary 

 veins, i, empty themselves into the left auricle, and this into the 

 left ventricle ; a similar valve to that of the right side, the valvula 

 hicuspis, preventing any retrograde motion of the fluid into the 

 auricle. From the left ventricle the blood passes into the aorta, 

 which, bifurcating into g, the anterior, and h, the posterior aorta, 

 conducts it through the medium of the arteries branching off 

 from these vessels to all parts of the body. Semilunar valves are 

 also placed at the origin of the aorta from the heart, and serve a 

 like purpose to those existing in the pulmonary artery. The 

 arteries of the system in their course give off many branches, all of 

 which end in hair-like vessels, capillaries (see figs. 6 and 7), by 

 which the blood is appropriated to the maintenance of the several 

 tissues ; here it likewise undergoes a chemical change (which 

 we shall presently describe), and is afterwards returned to the 

 heart by the veins which unite and form the two cavse before 

 spoken of. This circulation of the blood is divided into the pul- 

 monic, or that which conveys it from the right to the left side of 

 the heart through the lungs, and the systemic, or that which takes 

 it from the left to the right side, through the arteries and veins 

 of the system. The contraction of each auricle is simultaneous, 

 and precedes a little that of the ventricles, which likewise con- 

 tract together. This action of the heart produces the pulse, and 

 the number of its pulsations within a given time materially assists 

 the surgeon in arriving at a correct diagnosis when an animal is 

 suffering from disease. 



Having explained " the general round of circulation," we shall 

 add some further remarks on the arteries and veins, and after- 

 wards speak of the chemical changes of blood. These vessels 

 are represented in fig. 5, the artery being marked a, and the 

 vein B : it will also be seen that their inner structure is exposed 

 by a section being carried through their coats. 



It has already been stated that the arteries arising from the 

 heart are two, namely, the aorta and the pulmonary, and that valves 

 are placed at their origin to prevent a retrograde movement of the 

 blood ; from which it will be inferred that these vessels are not 

 mere passive conduits for the fluid. The amount of their action 



