238 ESSENTIALS OF PHYSIOLOGY. 



each beat, and thus to assist the vaso-motor centre in raising the blood 

 pressure. 



The pressure of the contents upon the wall of the skull is equal to 

 that within the capillaries of the brain. It cannot be greater than the 

 capillary pressure, since in these circumstances the capillaries would 

 collapse and the flow of blood through the brain would cease. The 

 intra-cranial pressure can be raised by any obstruction to the escape of 

 blood from the cerebral veins, or by the presence within the skull of a 

 foreign body, such as a blood clot. In the latter case the pressure 

 within the skull may rise sufficiently to compress the capillaries or even 

 actually to obliterate them. Such compression, which inevitably 

 diminishes the blood supply to the brain, may cause loss of conscious- 

 ness and other serious symptoms. 



SUMMARY OF THE ESSENTIAL FEATURES OF 

 THE CIRCULATION. 



The most important functions of the circulatory mechanism are (1) 

 to maintain a continuous slow stream of blood through the capillaries 

 so as to provide the most favourable conditions for the passage of oxygen 

 and nutritive material from the blood to the tissues and of waste pro- 

 ducts into the blood ; and (2) to keep the arterial pressure at such a 

 height as to ensure an adequate supply of blood to all parts of the body. 

 The carrying out of these functions in response to the varying needs of 

 the tissues and to stimuli reaching the body from the. outer world is 

 chiefly effected through the nervous system, which controls the rate of 

 the heart and the calibre of the arterioles. In the attainment of this 

 end, the heart and arterioles are made to co-operate with each other. 

 For instance, if the peripheral resistance becomes very great, the rise in 

 blood pressure stimulates the vagus centre, the heart beats more slowly, 

 and the pressure falls to its normal level (Marey's law). Again, if the 

 heart is beating against a high peripheral resistance, its work is 

 lightened by impulses passing along the depressor nerve, which reflexly 

 dimmish this resistance. If the blood pressure falls, it is restored to 

 the normal level by increased activity of the vaso-motor centre, which is 

 assisted in many cases by an increased output of blood from the heart. 



These regulative mechanisms depend for their efficiency upon the 

 nutrition of the heart itself, and upon the maintenance of its normal 

 rhythm. Broadly speaking, the rate of the heart is determined by the 

 influence of the nervous system, and the force of the beat by the state 

 of nutrition of its muscular fibres. The latter depends almost entirely 

 upon the amount and character of the blood supplied to the heart 

 by the coronary vessels ; if its nutrition is impaired the heart beats 



