464 PHYSIOLOGY 



not deprive the blood of any very large amount of the oxygen which it con- 

 tains. Nor does it seem probable that the brain requires a large supply 

 of food material. It must be remembered that in all parts of the brain 

 a peri-vascular lymphatic intervenes between the capillary and the brain 

 tissue. Since these ' lymphatics ' communicate with the subarachnoid 

 space they must contain a fluid which differs little if at all from the compo- 

 sition of the cerebro-spinal fluid obtained from the subarachnoid space. The 

 nutrient fluid of the brain is therefore practically salt solution with a trace 

 of sugar and possible minute traces of amino acids. 



Our study of the events which accompany the propagation of a nervous 

 impulse down a nerve fibre has prepared us for the conclusion that very little 

 energy is involved in ordinary nerve activity. It is true that extreme fatigue 

 causes changes in the initial granules of the nerve cells and is therefore asso- 

 ciated with the using up of some material constituent. But even though 

 material changes in the nerve-cells and in the synapses may be larger in 

 amount than those in nerve fibres, they are probably not to be compared 

 in extent with those taking place in a contracting muscle or in an active liver 

 cell. 



THE CEREBRAL CIRCULATION 



In all higher animals the brain is enclosed in a rigid case formed by the 

 bony cranium. In the child, before the cranial vault is fully ossified, part 

 of this vault consists of membrane, known as the anterior fontanelle. It 

 is easy to see that the fontanelle pulsates with each heart-beat as well 

 as with rise of venous pressure, such as that produced during strong 

 expiratory efforts. When ossification is complete, such alterations in the 

 volume of the cranial contents are impossible. And yet the pressure in 

 the arteries within the cranium must be still pulsatile, the rise of pressure at 

 each heart-beat must make the arteries expand, but room for this expansion 

 has to be found by contraction of some other part of the cranial contents. 

 We find that each arterial beat is associated with a corresponding expulsion 

 of some of the contents of the veins and a contraction of these vessels. If, 

 for instance, a cannula be introduced through the occipital bone into the 

 torcular Herophili, the venous blood is seen to pulsate and to be pressed out 

 with each beat of the heart. If there is a rise of arterial pressure, although 

 the arteries may expand somewhat at the expense of the veins, there can be 

 no dilatation of the whole organ. The only effect of the rise of pressure 

 will be to cause an increased pressure fall in the cranial vascular system, and 

 therefore augmented velocity of flow through the system. A prolonged rise 

 of pressure may cause a certain amount of dilatation of the vessels, but only 

 at the expense of the cerebro-spinal fluid. Since this is only small in amount, 

 any expansion of the brain due to vascular causes must be very limited. 



BRAIN PRESSURE. If by means of a trephine an opening be made into 

 the cranial vault, the brain bulges into the opening. By screwing a tube 

 covered with a membrane into the trephine opening, we can find the pressure 

 necessary to force the brain back to its previous position. This is known 

 as the brain pressure and is approximately equal, as might be expected, to the 



