674 REFLEX STIMULATION OF THE VASO-MOTOR CENTRE. 



stronger contraction of the arteries, and consequently an increase of the arterial 

 blood-pressure. These are called "pressor " fibres. Conversely, there are other 

 fibres whose stimulation reflexly diminishes the excitability of the vaso-motor 

 centre. These act as reflex inhibitory nerves on the centre, and are known as 

 " depressor " fibres. 



Pressor, or excito-vaso-motor nerves, have already been referred to in connection 

 with the superior and inferior laryngeal nerves ( 352, 12, a), in the trigeminus, 

 which, when stimulated directly ( 317), causes a pressor action, as well as when 

 stimulating vapours are blown into the nostrils (Hering and Kratschmer). ^ [The 

 rise of the blood-pressure in this case, however, is accompanied by a change in the 

 character of the hearts beat and in the respirations. Rutherford has shown that 

 in the rabbit the vapour of chloroform, ether, amyl nitrite, acetic acid, or ammonia 

 held before the nose of a rabbit, greatly retards or even arrests the heart's action, 

 and the same is true if the nostrils be closed by the hand. This arrest does not 

 occur if the trachea be opened, and Rutherford regards the result as due not to the 

 stimulation of the sensory fibres of the trigeminus, but to the state of the blood 

 acting on the cardio-inhibitory nerve apparatus.] Hubert and Roever found pressor 

 fibres in the cervical sympathetic ; S. Mayer and Pribram found that mechanical 

 stimulation of the stomach, especially of its serosa, caused pressor effects ( 352, 

 12, <). According to Loven, the first effect of stimulating every sensory nerve is a 

 pressor action. 



[If a dog be poisoned with curara, and the central end of one sciatic nerve be 

 stimulated, there is a great and steady rise of the blood-pressure, chiefly owing to 

 the contraction of the abdominal blood-vessels, and at the same time there is no 

 change in the heart-beat. If, however, the animal be poisoned with chloral, there 

 is a fall of the blood-pressure resembling a depressor effect.] 



< >. Xnumann found that weak, electrical stimulation of the skin caused at first contraction 

 <>f the blood-vessels, especially of the mesentery, lungs, and the web, with simultaneous excite- 

 ment of the cardiac activity and acceleration of the circulation (frog). Strong stimuli, however, 

 had an opposite effect, i.e., a depressor effect, with simultaneous decrease of the cardiac activity. 

 Griitzner and Heidenhain found that contact with the skin caused a pressor effect, while pain- 

 ful impressions produced no effect. The application of heat and cold to the skin produces 

 reflexly a change in the lumen of the blood-vessels and in the cardiac activity (Ebhrig, Winter- 

 nitz). Pinching the skin causes contraction of the vessels of the pia mater of the rabbit 

 (Sch idler), and the same result was produced by a warm bath, while cold dilated the vessels. 

 These results are due partly to pressor and partly to depressor effects, but the chief cause of the 

 dilatation of the blood-vessels is the increased blood- pressure due to the cold constricting the 

 cutaneous vessels. Heat, of course, has the opposite effect. In man, most stimuli applied to 

 sensory nerves produce an effect : feeble cutaneous stimuli, tickling (even unpleasant odours, 

 bitter or acid tastes, optical and acoustic stimuli) at the parts where they are applied, cause a fall 

 of the cutaneous temperature, and diminution of the volume of the corresponding limb, some- 

 times increase of the general blood-pressure and change of the heart-beat. The opposite effects 

 are produced by painful stimulation, the action of heat (and even by pleasant odours and sweet 

 tastes). The former cause simultaneously dilatation of the cerebral vessels and increase 

 the vascular contents of the skull, the latter cause the opposite results (Isiominow and 

 Tarchanoff). 



Depressor fibres, i.e., fibres whose stimulation diminishes the activity of the 

 vaso-motor centre, are present in many nerves. They are specially numerous in 

 the superior cardiac branch of the vagus, which is known as the depressor nerve 

 ( 352, 6). The trunk of the vagus below the latter also contains depressor fibres 

 (v. Bezold), as well as the pulmonary fibres (dog). The latter also act as depressors, 

 during strong expiratory efforts ( 74) ; while Hering found that inflating the lungs 

 (to 50 mm. Hg pressure) caused a fall of the blood-pressure (and also accelerated 

 the heart beats 369, II. j. Stimulation of the central end of sensory nerves, 

 especially when it is intense and long-continued, causes dilatation of the blood- 

 vessels in the area supplied by them {Loven). According to Latschenberger and 

 Deahna, all sensory nerves contain both pressor and depressor fibres. 



