292 FUNCTIONS OF THE NERVOUS SYSTEM. 



of all the actions of the Spinal Cord; and we find its maintenance, in spite 

 of accident or disease of the spine, remarkably provided for, in the location of 

 the centre of the respiratory movements, which occupies a position where it 

 receives the greatest possible amount of protection. The supply of the di- 

 gestive apparatus, again, is immediately dependent upon the Spinal system ; 

 and this, being another essential function, has its centre equally protected. 

 The outlets of the cavities are also controlled by the Spinal system; but this 

 control, although essential to the comfort of life, is less necessary to its main- 

 tenance ; and we find it dependent upon a portion of the Cord, which is 

 more liable to lose its powers by disease or injury. It is possible, as will 

 hereafter be shown, that several actions, which are at first voluntary, may be 

 effected, when so frequently performed as to ^become habitual, through the 

 medium of the Spinal system ; of this kind seem to be the movements of 

 locomotion, which are continued involuntarily, when the whole attention of 

 the mind is given to other objects, but which the Will can check at any time. 

 We shall commence our particular survey of the Reflex movements in Man, 

 with the consideration of those of Respiration, which are well adapted for 

 illustrating their general character. 



374. Respiratory Movements. The centre of these is the upper part of 

 the Medulla Oblongata; into this may be traced the excitor nerves, that con- 

 vey the stimulus on which the movements are dependent; and from it pro- 

 ceed, either directly or indirectly, the motor nerves by which they are carried 

 into effect. The chief Excitor of the respiratory movements is unquestion- 

 ably the Par Vagum. When this is divided on both sides, according to the 

 experiments of Dr. Reid,* the number of respiratory movements is considera- 

 bly diminished, usually about one-half. Now if this nerve excites the motions 

 of respiration by its powerful action in producing sensation, we should ex- 

 pect to find its trunk endowed with considerable sensibility, which is not the 

 case; for all experimenters agree in stating that, when its trunk is pinched or 

 pricked, the animal does not exhibit signs of pain nearly so acute, as when 

 the trunks of the ordinary spinal nerves, or of the fifth pair, are subjected to 

 similar treatment. It cannot be questioned, however, that its power as an 

 excitor of respiration is very great; since, besides the fact of the diminution 

 in the number of inspirations which occurs immediately on section of it, 

 irritation of its trunk in the neck is instantly followed by an act of inspira- 

 tion. It is evident that this power must arise from impressions made upon 

 its peripheral extremities. The impression is probably due to the presence 

 of venous blood in the capillaries of the lungs; or, as Dr. M. Hall thinks, to 

 the presence of carbonic acid in the air-cells. Either or both may be true. 

 The Pneumogastric nerve, however, is not the only excitor of the respiratory 

 movements ; since, when the nerve is cut on each side, they still continue. 

 Dr. Reid has satisfactorily shown the statement of many experimenters, that 

 the inspirations are increased in frequency after this operation, to be erroneous ; 

 this idea having originated in their very prolonged and laborious character. 

 The removal of the Encephalon, also, diminishes the frequency of the respi- 

 ratory movements, whether it be performed before or after the section of the 

 Vagi. Dr. Reid found that, in a kitten of a day old, in which the inspira- 

 tions were 100 per minute, they fell to 40 when the Encephalon was re- 

 moved ; and on subsequently cutting the Pneumogas tries, the number of 

 inspirations instantly fell to between 3 and 4 in the minute, and continued so 

 for some time. Hence it appears that the respiratory movements are partly 

 dependent upon sensation, and a motor influence excited by it; and this may 

 also be learned from the prolonged and laborious character of the inspirations 



* Edinb. Mecl. and Surg. Journ., vol. li. 



