4/2 PRINCIPLES OF GENERAL PHYSIOLOGY 



especially the need for oxygen. The question of the respiratory exchange of the 

 central nervous system has not been thoroughly investigated. Leonard Hill and 

 Nabarro (1895) found the oxygen consumption and the carbon dioxide evolution to 

 be considerably less than that of muscle, as would be expected. The rate of the 

 blood current was not taken especially into account in these experiments, so that it 

 is difficult to compare the absolute respiration of the brain with that of other organs. 

 The dark colour of the blood in the cerebral veins suggests fairly considerable 

 consumption of oxygen and the immediate loss of consciousness, produced in man by 

 cessation of the normal supply of arterial blood, shows the dependence on constant 

 supply of oxygen. If the actual consumption is small, this latter fact shows that 

 it must be present with a high tension in order to act efficiently. 



In the work of Alexander and Cserna (1913) the rate of flow was determined and the 

 conclusion was arrived at that the small values of Hill and Nabarro were due to the animals 

 being narcotised. If allowed to escape from the influence of deep anaesthesia, the oxygen 

 consumption was found to be very considerable, namely, 0'36 c.c. per gram per minute. If 

 this is not a misprint for 0*036 c.c., it seems to throw some doubt on the accuracy of the 

 method used, since Barcroft and Dixon only obtained O'Oll for the heart muscle and Barcroft 

 0'028 for the salivary glands (see Bancroft's article, 1908, p. 757). 



It has not been possible to prove satisfactorily the existence of any other 

 metabolic change in nervous tissue. Of course, the products of the breaking down 

 of lecithin and other constituents might be found in the blood, or elsewhere, when 

 extensive degeneration processes are taking place. 



THE NERVE NETWORK 



It is obvious that there must be physiological continuity between constituent 

 units of the nerve centres in all cases. In all animals above the Coslenterate con- 

 dition, however, there is not direct structural protoplasmic continuity between the 

 neurones ; we have already spoken of the syiiaptic membrane intervening, and we 

 shall have to discuss its properties presently. In the Coelenterates, and possibly 

 in certain parts of the higher animals, there is a kind of nervous tissue, possessing 

 certain properties of central nature, in which a direct anatomical connection 

 appears to exist between the various nerve fibres and cells. This is associated 

 with the power to conduct impulses in all directions, as we saw in the case of the 

 swimming movements of the jelly-fish. Here the ability to perform co-ordinated 

 movements depends upon the refractory phase, so that impulses arriving at the 

 centre when in a state of activity are inoperative. Further details may be found 

 in the paper by Romanes (1876) and in Bethe's book (1903, pp. 78-124). 



Von Uexkiill (1909, p. 81) finds it necessary to introduce the conception of "interrupter"' 

 or " representant " as intervening between the nerve net and the muscle fibre, whose property 

 it is to accumulate and give out "excitation." This excitation flows from a place of higher 

 pressure in the network to one of lower pressure, so that a stretched muscle becomes 

 stimulated and vice versa (see also p. 135 of von Uexkiill's book). 



In higher invertebrates, such as the earthworm, the neuropile of the central 

 nervous system has been stated to consist of a network of the processes of the 

 afferent and efferent cells. It is a matter of much difficulty to be certain as to 

 whether there is direct anatomical continuity, and, in fact, Retzius (1892, p. 14) 

 says that connection is by contact. 



Even in vertebrate animals, we find in peripheral parts nervous interlacings 

 which appear to be networks. A figure of such an arrangement in the palate of 

 the frog is to be found on p. 79 of Bethe's book (1903). Similar structures exist 

 on the walls of blood vessels, and in connection with smooth muscle generally 

 (see Fig. 109, page 402, after Retzius, 1892). With regard to these "networks," 

 although there are local thickenings to be seen in stained preparations, especially 

 at places where branches are given off, there is no evidence that they possess the 

 properties of centres, such as that of reflex action. Moreover, if the figures given 

 by Retzius (1892) be looked at carefully, the impression is given that there is no 

 anastomosis between branches of different nerve fibres. At the same time, in such 

 cases as those of the vasornotor nerves, there is no necessity for the separate 

 stimulation of different muscle cells, as in the delicate adjustments of voluntary 



