THE CIRCULATION OF THE BLOOD AND LYMPH 69 



been modified to act as a pump for driving the blood in a 

 definite direction. Morphologically it is a bloodvessel ; and 

 the physiological property of rhythmical contraction which 

 belongs to the muscle of the heart in so eminent a degree 

 is, as has been mentioned (p. 67), an endowment of blood- 

 vessels in many animals that possess no localized heart. 

 Even in some mammals contractile bloodvessels occur ; the 

 veins of the bat's wing, for example, beat with a regular 

 rhythm, and perform the function of accessory hearts. 



The whole vascular system is lined with a single layer 

 of endothelial cells. In the capillaries nothing else is 

 present ; the endothelial layer forms the whole thickness of 

 the wall. In young animals, at any rate, the endothelial 

 cells of the capillaries are capable of contracting when 

 stimulated ; and changes in the calibre of these vessels can 

 be brought about in this way. The walls of the arteries and 

 veins are chiefly made up of two kinds of tissue, which 

 render them distensible and elastic: non-striped muscular 

 fibres and yellow elastic fibres. The muscular fibres are 

 mainly arranged as a circular middle coat, which, especially 

 in the smaller arteries, is relatively thick. One conspicuous 

 layer of elastic fibres marks the boundary between the middle 

 and inner coats. - In the larger arteries elastic laminae are 

 also scattered freely among the muscular fibres of the middle 

 coat. The outer coat is composed chiefly of ordinary con- 

 nective tissue. The veins differ from the arteries in having 

 thinner walls, with the layers less distinctly marked, and 

 containing a smaller proportion of non-striped muscle and 

 elastic tissue ; although in some veins, those of the pregnant 

 uterus, for instance, and the cardiac ends of the large 

 thoracic veins, there is a great development of muscular 

 tissue. Further, and this is of prime physiological import- 

 ance, valves are present in many veins. These are semilunar 

 folds of the internal coat projecting into the lumen in such a 

 direction as to favour the flow of blood towards the heart, 

 but to check its return. In some veins, as the venae cavae, 

 the pulmonary veins, the veins of most internal organs, and 

 of bone, there are no valves ; in the portal system they are 

 rudimentary in man and the great majority of mammals. 



