464 PHYSIOLOGY 



deprivation of oxygen. Although so susceptible to slight deprivation 

 of oxygen it does not seem that the brain tissues have a very rapid gaseous 

 metabolism ; that is, they need oxygen supply at high tension but do 

 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 tho 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 possibly 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 Nissl 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 t In- 

 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 expira- 

 tory efforts. When ossification is complete, such alterations in the volume 

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

 wit li in. 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 in 

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

 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 nt 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 he, 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. 1 1 l\ means of a trephine an opening be made into 



