62 PHYSIOLOGY OF FARM ANIMALS [CH. 



oxygen. The puritied or oxygenated blood then traverses the 

 pulmonary veins which convey it to the left auricle; from this 

 chamber it goes through the mitral valve to the left ventricle, 

 and thence through the aorta being driven through the semi- 

 lunar valves. The aorta gives off numerous arteries, which 

 further divide many times and convey the blood to all the organs 

 and parts of the body. The smallest arteries give off still smaller 

 vessels called capillaries, and having a diameter hardly greater 

 than that of a red corpuscle. It was in the capillaries of the 

 web of the frog's foot that Harvey first demonstrated the fact that 

 the blood circulates, and the observation may be repeated by any- 

 one who examines this organ in a living frog with the low power 

 under the microscope. The capillaries reunite together to form 

 the veins through which the blood is conveyed back to the heart. 

 The veins form an elaborate branching system resembling ^hat 

 of the arteries, the smallest veins being those formed by the 

 junction of capillaries. 



The arteries have thicker walls than the veins, and excepting 

 the aorta and the pulmonary artery do not possess valves. 

 The veins on the other hand may have valves which admit of 

 the blood passing in the direction of the heart but not in the 

 opposite direction. The Avail of an artery consists typically of 

 three coats, the mner consisting of elastic tissues lining the lumen, 

 the middle of unstriated muscle fibres and the outer of fibrous 

 tissues. The wall of a vein contains less muscular and elastic 

 tissue but is otherwise not dissimilar to that of an artery. 



The Lymph. A capillary has a still thinner wall than a vein, 

 and through this wall the blood plasma transudes forming the 

 lymph which is contained in the loose connective tissue and bathes 

 the cells of the body. It is this intimate relation between the 

 lymph and the tissues which admits of the process known as 

 ' tissue respiration ' (that is to say, the gaseous exchange between 

 the tissues and the blood) and of nourishment being supplied 

 from the blood to the tissues, and of waste products being re- 

 moved. From the unenclosed spaces in the tissue the lymph is 

 collected in thin- walled vessels which join together forming the 

 lymphatics. These vessels, which like the veins may possess 

 valves, communicate eventually with the thoracic duct or with 

 the left lymphatic trunk which open into the large veins near 

 the heart as already described at the end of the last chapter. 



