284 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



the seat and mode of the activities of the central nervous system which 

 determine the respiratory movements. 



The respiratory muscles receive their nerve supply from various 

 parts of the cerebro-spinal axis. Thus the muscles moving the alae 

 nasi are innervated from the seventh nerve ; the muscles opening the 

 glottis, from the accessory part of the vagus ; the diaphragm, from the 

 cervical cord by the fourth and fifth cervical nerves ; and the intercostal 

 muscles, from the whole of the dorsal cord. From what we know of the 

 anatomy of the cord, we may affirm almost certainly that the motor 

 fibres going to the inspiratory muscles are derived from cells in the 

 grey matter of the anterior cornua of the cord, or from the corresponding 

 part of the grey matter in the medulla oblongata. All these cells 

 represent, therefore, centres for the innervation of the inspiratory 

 muscles, and any contraction of these muscles can only be brought 

 about during life through the intermediation of these cells. Under 

 normal conditions, the motor cells of the cord are excited to activity by 

 the impression on them of nervous impulses derived from sensory fibres 

 which have entered the cord by the sensory roots, or from fibres 

 originating in cells situated in the higher parts of the nervous system, e.g. 

 pyramidal tract fibres. 



The normal performance of the respiratory movements is dependent, 

 then, upon a rhythmic and co-ordinate activity of a number of cells 

 extended over a considerable length of the medulla oblongata and spinal 

 cord. The question we have to discuss is the manner in which the 

 activities of these cells are roused and co-ordinated. Are they so con- 

 stituted that their normal metabolism leads to rhythmic automatic dis- 

 charges, or is their activity excited by changes in the blood passing 

 to them or by afferent impulses arriving from the periphery ? Is 

 the co-ordination of their activities dependent on intercentral com- 

 munications between the motor cells at various levels of the cord, or 

 are they harmonised in obedience to impulses proceeding to them from 

 higher parts of the central nervous system ? In the latter case, can we 

 by experiment localise any part of the nervous system which carries 

 out this function of origination and co-ordination, and so plays the part 

 of a respiratory centre ? 



The first attempts to answer these questions by systematic experi- 

 ments were made by Legallois. 1 His investigations were amplified and 

 confirmed by Flourens. 2 These experiments were as follows: If the 

 spinal cord be divided on a level with the seventh cervical nerve, 

 respiration continues, but is carried out solely by means of diaphragmatic 

 contractions, the chest walls remaining motionless or being moved 

 passively by the contractions of the diaphragm. If the spinal cord be 

 divided at the third cervical nerve (i.e. above the origin of the phrenic 

 nerves), the respiratory movements of the thorax either cease entirely, 

 or are confined to spasmodic elevations of its upper part, induced by 

 contractions of the sterno-mastoid and trapezius muscles. The animal 

 therefore dies of asphyxia if artificial respiration be not resorted to. 

 Kespiratory movements, although not effectual in aerating the lungs, 

 are still to be observed in the laryngeal muscles, and in the muscles of 

 the face. These experiments seem to indicate that the spinal centres 



1 "(Euvres completes," Paris, 1824, tome i. 



" Recherches expe'rimentales sur les proprietes et fonctions du systeme nerveux," 

 2me edition, Paris, 1842 ; Compt. rend. Acad. d. sc., Paris, 1858, tome xlvii. p. 803. 



