26 



CIRCULATION. 



This involves the consideration of the heart, arteries, and veins. 



Blood. 



The fluid which enriches all li\ang structures, being the medium 

 by which nutritive material is convej^ed to the solid tissues. It 

 is an opaque, thickish, clammy fluid with a peculiar odor, sickly 

 saline taste, and alkaline reaction. Its color varies in different 

 parts of the same animal, that in the arteries being a bright red 

 or scarlet, while that in the veins is of a dark purplish hue. 



Heai't. 



The heart is a hollow organ of involuntary muscular structure, 

 situated between the lungs, in the thoracic cavity; its average 

 weight is about Gi^ pounds. 



The heart is divided into two parts, right and left, each part 

 containing two cavities, one above the other, which communicate 

 by valvular openings. The heart acts as a force pump for the 

 blood, forcing the impure from the right side of the heart through 

 the pulmonary artery into the lungs, where the blood gives off 

 carbonic acid gas and takes up oxygen; the purified blood returns 

 through the pulmonary vein to the opposite (left) side of the 

 heart, and is then forced by it into the arteries, which carry it 

 to all parts of the body, giving nourishment to the tissues and 

 taking up waste material. This impure blood is returned to the 

 right side of the heart by the veins, thus completing the course 

 of the circulation. 



The smaller arteries terminate in a system of minute vessels — 

 the capillaries— which are situated between the termination of 

 the arteries and the commencement of the veins. Their average 

 diameter is about jo^o of an inch. 



Circulation of the extremities. 



The humeral artery. — This artery descends along the inner side 

 of the humerus; just above the elbow joint it divides into ante- 

 rior and posterior radial arteries. The anterior radial descends 

 over the anterior surface of the elbow joint, passes down in front 

 of the radius and approaches the knee below the extensor 2^edis 

 muscle, where it divides into numerous branches, supplying 



