416 



PRINCIPLES OF GENERAL PHYSIOLOGY 



escape (see my paper, 1908, 3, p. 272). Hence, if this cervical sympathetic nerve, 

 conveying constrictor impulses to the gland vessels, had been intact, explanation 

 would be simple, namely, by reflex excitation of vaso-constrictor nerves. But, 

 in point of fact, the vaso-dilator fibres of the chorda tympani were the only 

 vascular nerves left intact, so that the effect must have been produced through 

 these and indeed by inhibiting a previously existing tonic state of an inhibitory 

 centre. It may be noted that one inhibition acts on muscle, the other on 

 nerve cells. 



It is shown by the work of Pavlov on "Conditioned Reflexes" (see p. 506) 

 that similar phenomena play an important part in the mechanism of the IKTM- 

 centres themselves. Suppose that a motor nerve centre is inhibited by the action 

 upon it of an inhibiting nerve, acting through an intermediate neurone. In other 

 words, the state into which this intermediate neurone is put by the stimulation 

 of a particular afferent nerve is such as to give rise to inhibition of the motor 

 neurone. Now it is quite possible that another nerve, acting on this intermediate 



neurone, might 

 inhibit it ; in 

 which case, the 

 motor neurone 

 previously kept 

 in check by the 

 intermediate 

 neurone would 

 be set free, and 

 apparently ex- 

 cited by a nerve 

 which was really 

 inhibiting an- 

 other neurone. 

 The state of ex- 

 citation of our 

 motor neurone 

 is, of course, 

 assumed to arise 

 from some inde- 

 pendent cause, 

 inherent, reflex, 

 or chemical. We 

 shall see pre- 

 sently that an 

 inhibitory nerve 

 can antagonise a 



chemical excitation as well as a nervous one, and, perhaps, the distinction ought 

 not to be made in this form, since the ultimate process is probably identical in 

 both cases, as remarked above (page 344). 



"Shock" and " Decerebrate Rigidity" When the spinal cord is cut across^ 

 it is found that the portion posterior to the section remains for some time in 

 a state of diminished excitability, so that no reflexes can be obtained from it. 

 This state of " shock " is gradually recovered from. It has been supposed to 

 be due to some effect of the injury, but Trendelenburg (1910) has made some 

 experiments which indicate that this is not the explanation. Instead of cutting 

 through the cord, he abolished its power of conduction by local cooling to a point 

 just above its freezing point. The experiments were done on the rabbit, since 

 the blood supply of the cord in this animal is such as to preclude the possibility 

 of cooled blood passing to the posterior part of the cord and affecting its 

 excitability directly. It was found that functional separation of the lumbar 

 cord from the higher parts of the central nervous system could be made without 

 any excitation. Notwithstanding this, the reflex excitability of the lumbar cord 

 was abolished, but returned again when the temperature of the cooled segments 



Fio. 124. CENTRAL INHIBITION OF EXCITATION OF INHIBITORY NERVES, 



THAT IS, OF NERVES WHICH RELAX TONCS IN PERIPHERAL MUSCLHS. 



INHIBITION OF THE VASO-DILATOR CENTRE. Rabbit. 



Upper tracing arterial pressure. Zero at level of upper signal Scale in millimetres. 



Top signal drops of blood from vein of submaxillary gland. 



Middle signal two stimulations of the cenlral end of the vagus of the opposite side 

 (pressor nerve). 



Bottom signal time in ten seconds. 



The vagi, sympathetic, and depressor nerves were cut on both sides, so that the gland 

 vessels were supplied only by dilator nerves in the chorda tympani nerve. 



Stimulation of the vagus produced no rise of blood pressure, because the animal was 

 eviscerated and the aorta was connected with an automatic arrangement, by which 

 rise of pressure was prevented. We note that although there was no vaso-con- 

 strictor supply, yet vascular constriction was produced in the submaxillary gland, 

 as shown by the longer intervals between the drops of blood escaping. Since the 

 only vasomotor nerves were dilators, constriction could only be caused by in- 

 hibiting the impulses from the centre which kept these nerves in a state of tonic 

 stimulation. 



