470 VETERINARY PHYSIOLOGY 



jDulp, and it is driven on by the alternate contraction and 

 relaxation of the non-striped muscle in the capsule and 

 trabeculae (see p. 215). 



5. Bone-Marrow, — The tissue is surrounded by rigid bony 

 walls, and the amount of blood can vary only slightly. The 

 circulation is through sinusoid capillaries. 



D. EXTRA-CARDIAC FACTORS MAINTAINING 



CIRCULATION. 



The central pump, the heart, is not the only factor main- 

 taining the flow of blood through the vessels (fig. 196). 



1. Movements of Respiration. — (i.) The thorax, in the 

 movements of respiration, is a suction pump of considerable 

 power, which draws blood into the heart during inspiration. 

 The auricles may be regarded as the cisterns of the heart, 

 the abdominal 'blood-vessels as the grreat blood reservoir, 

 and the diaphragm, contracting in inspiration, presses the 

 blood from this reservoir up into the thorax and heart. 



(ii.) Expiration also helps, for the blood, which has filled 

 the vessels of the lungs in inspiration, is driven on into the 

 left side of the heart in expiration. The blood is thus forced 

 on into the arteries. The respiratory movements apparently 

 play a great ]3art in maintaining the circulation when the 

 heart has undergone extensive calcareous derreneration. 



2, Intermittent Muscular Exercise, — This acts in three 

 ways: (1) by increasing the respiratory movements; (2) 

 by augmenting the action of the heart ; (3) by the contract- 

 ing and relaxing muscles pressing on the blood-vessels, and 

 so forcing the blood onwards into the veins and to the heart, 

 back-flow in the veins being prevented by the valves ; 

 (4) by the increased venous filling of the heart leading to 

 stronger contractions fp. 417), and reflexly to acceleration 

 (p. 421). 



The arterial blood pressure is thus raised and the intra- 

 cranial circulation accelerated, so that more blood is sent to 

 the brain. Too marked a rise of pressure during such 

 exercise is prevented by dilatation of the arterioles throughout 

 the body. 



