RESPIRATORY MOVEMENTS. 139 



tions requisite for the maintenance of the various Organic processes. Thus, 

 the aeration of the blood, which takes place whenever that fluid is pieced in 

 relation with the atmosphere, can only be carried on, by the regular exchange 

 of the small quantity of the gas contained in the lungs ; if this cease, the cir- 

 culation is soon brought to a stand, and loss of vitality of the whole system 

 speedily results. Hence this is the most constantly necessary of all the actions 

 of the Spinal Cord; and w r e find its maintenance, in spite of accident or disease 

 of the spine, remarkably provided for, in the location of the centre of the respira- 

 tory movements, which occupies a position where it receives the greatest 

 possible amount of protection. The supply of the digestive 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 function, although essential 

 to the comfort of life, is less necessary to its maintenance ; 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 per- 

 formed as to become habitual, through the medium of the Spinal system : of 

 this kind seem to be the movements of locomotion, which are continued invo- 

 luntary, 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. 



* ! IX. Respiratory Movements. 



184. The centre of the Respiratory movements is the upper part of the 

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

 the stimulus on which the movements are dependent; and from it proceed, 

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

 effect. The chief Excitor of the respiratory movements is unquestionably the 

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

 ments of Dr. Reid,* the number of respiratory movements is considerably 

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

 respiration by its powerful action in producing sensation, we should expect 

 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 

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

 the number of inspirations which occurs immediately on section of it, irrita- 

 tion of its trunk, in the neck is instantly followed by an act of inspiration. It 

 is evident that this power must arise from impressions made upon its peri- 

 pheral 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 pre- 

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

 Pneifmogastric nerve, however, is not the only exciter 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 erro- 

 neous ; this idea having originated in their very prolonged and laborious cha- 

 racter. The removal of the Encephalon, also, diminishes the frequency of 



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



