352 PHYSIOLOGY 



sacral cord. In the second place, the ready production of fatigue 

 of the nervous system points to a considerable using up of material 

 as a condition of the passage of nerve impulses. In many instances, 

 moreover, an infinitesimal stimulus travelling up a few nerve fibres may 

 excite widespread activity of the whole central nervous system with 

 the discharge of impulses along practically every nerve of the body. 

 Thus the presence of a crumb on the larynx w r ill excite impulses 

 travelling up the superior laryngeal nerve, which in themselves can 

 involve but little expenditure of energy. The result, however, of their 

 arrival at the central nervous system is the discharge of impulses along 

 the motor nerves causing spasmodic contractions of almost every 

 muscle in the body. It seems beyond doubt then that energy is evolved 

 in the central nervous system as a result of metabolic changes, and 

 that energy may be added to impulses passing through the central 

 nervous system, which therefore acts as a relay of force. But this 

 activity does not necessarily require the presence or co-operation of 

 nucleated cells. In dealing with the nature of a nerve impulse we had 

 reason to conclude that there may be an actual, though minimal, ex- 

 penditure of energy by the axis cylinder with the passage of each nerve 

 impulse. The non- nucleated parts of a cell, whether the axon or the cell 

 body, are equally capable of this evolution of energy, and we might con- 

 ceive therefore of a nervous system which, existing for a few days, might 

 act as a normal reflex centre in the entire absence of the nucleated 

 cell bodies. This conception has been realised by Bethe in an experi- 

 ment on the crab (Carcinus rnenas). In this animal the reflex move- 

 ments of the tentacle are carried out by a ganglion situated at its base. 

 As in the other Crustacea, the cell bodies in this ganglion lie outside 

 the mass of neuro-fibrils in the centre, forming a sort of capsule (Fig. 

 153). Bethe was able, under the dissecting microscope, to remove 

 the cell bodies without interfering with the nerves entering or leaving 

 the central mass of fibrils. All the nerve processes with their connec- 

 tions were therefore left intact. In animals, operated in this way, 

 Bethe found that for two or three days the tentacle reacted normally 

 to stimuli applied to its surface. The reflex functions of the ganglion 

 were not in any way affected by the removal of the nucleated bodies 

 of the cells. A similar experiment would be impossible in the central 

 nervous system of vertebrates, since impulses must of necessity pass 

 through the cell body on their way from the termination of one axon 

 to the beginning of the next. In the spinal root ganglion, however, most 

 of the cells lie on the surface. In the rabbit Steinach exposed a 

 posterior root ganglion, separating it from all its vascular supply, but 

 leaving its nervous attachments intact. The wound was opened every 

 day for the next few days and an instrument passed under the ganglion 

 so as to divide any newly forming vessels. As a, result of the deprivation 



