562 TEXT-BOOK OF PHYSIOLOGY. 



resulting in a further increase in the volume of the brain and in the intra- 

 cranial pressure. The reverse takes place in inspiration. 



It has been ascertained experimentally that the intra-cranial pressure 

 may vary considerably and consciousness still be preserved. Hill found it 

 to be 40 to 50 mm. of Hg. in the convulsions of strychnin poisoning and a 

 little less than zero in a patient standing erect. 



The Regulation of the Volume of Blood Entering the Brain. It is 

 generally believed that the cerebral vessels are not provided with vaso-motor 

 nerves. Every attempt to prove their existence either by physiologic or 

 histologic methods has thus far failed of convincing proof. In the absence 

 of vaso-motor nerves, the regulation of the circulation in the brain must neces- 

 sarily be dependent on changes affecting the arterial and venous pressures in 

 other regions of the body. 



The most effective factor in increasing or decreasing the blood-supply 

 to the brain resides in the power of the vaso-motor center to cause a contrac- 

 tion or dilatation of the cutaneous and splanchnic vessels. Thus if the 

 vaso-motor center declines in its tonus from any cause whatever, there is a 

 relaxation of the blood-vessels in one or both of these regions, an increase in 

 the volume of the blood flowing into them, and in consequence, a decrease 

 in the volume of the blood flowing through the brain. If on the contrary the 

 vaso-motor center is increased in its tonus, the reverse conditions prevail in 

 the cutaneous and splanchnic vessels and the quantity of blood flowing into 

 the brain is increased. Thus in an indirect way the vaso-motor center, by 

 bringing about a rise or a fall in the general arterial pressure, regulates the 

 blood-supply to the brain, and controls its amount in accordance with its 

 needs. 



Brain Activity. Brain activity is characterized by an active conscious- 

 ness, the development of sensations, ideas, feelings, and the exercise of 

 volitional power (which manifests in muscle movement) and is the result of a 

 physiologic condition of the body at large. For the manifestation of brain 

 activity it is essential that the irritability of the brain cells and more especially 

 of those composing in large measure the cerebral cortex be maintained at a 

 normal physiologic level, so that they may respond in the manner peculiar 

 to them to the action of nerve impulses transmitted through afferent nerves 

 from all regions of the body. Here as elsewhere throughout the body, the 

 irritability depends on, and is maintained by, the presence of blood flowing 

 into and out of the brain in varying quantity from moment to moment, with 

 a given velocity and under a definite pressure. So long as these conditions 

 are maintained in the strictly physiologic condition, so long will the brain 

 respond to stimuli by the development of sensations. The avenues through 

 which nerve impulses pass to the cortical cells are those beginning in the 

 special and general sense organs of the body in contact with the external 

 world, viz.: the eyes, ears, nose, tongue, and skin. The maintenance of 

 these structures in a strictly physiologic condition is also one of the essential 

 conditions for brain activity. 



Judging from the changes in the character and composition of the blood 

 which occur during its passage through the brain capillaries, there is coin- 

 cidently with brain activity an active metabolism, which eventuates, at the 

 end of a variable number of hours, in the decline of the irritability, a reduc- 



