THE REGULATION OF BREATHING 



297 



standstill of the diaphragm. These effects are abolished when the 

 vagi are cut. These results are interpreted as showing that the vagus 

 nerve terminates in two sets of nerve endings, one set stimulated by 

 stretching of the lung during inspiration inspiration-inhibiting the 

 other stimulated by collapse of the lung inspiration-inducing. 



Co-ordinating the action of all the skeletal muscles are " prop-io- 

 ceptive fibres," through which extension inhibits flexors, and flexion 

 inhibits extensors (see Fig. 409, p. 683). It is most probable that 

 each respiratory act should consist of the action of one set of 



h 



D 



FIG. 161. INSPIRATORY SPASM OF THE DIAPHRAGM PRODUCEDBY EXCITATION OF THK 

 VAGUS DURING THE PERIOD SHOWN BY THE SIGNAL a, b. (Fredericq and Nuel. > 



The down-stroke represents inspiration; the up-stroke expiration. 



muscles, and inhibition of the antagonists, co-ordinated by afferent 

 fibres. The vagus nerve endings in the lungs would then correspond 

 to nerve endings in joints. The phrenic nerves contain afferent 

 fibres from, as well as motor fibres to, the diaphragm. The evidence 

 so far is positive for the action of only one set of nerve endings 

 inhibiting inspiration and these only when the inspiration is 

 large beyond the normal tidal capacity but it may well be that the 

 string galvanometer is not a delicate enough instrument to indicate 

 normal gentle nerve impulses. We may conclude that while the 



FIG. 162. EXPIRATION SPASM OF THE DIAPHRAGM PRODUCED BY WEAK 

 STIMULATION OF THE VAGUS. (Fredericq and Nuel.) 



The down-stroke represents inspiration; the up-stroke expiration, 

 shows the duration of stimulation. 



The signal line 



chemical stimulus of acid in the blood is always present to induce 

 a new inspiration, the afferent fibres to the centre reflexly make 

 the muscles work smoothly and with perfect co-ordination. 



Stimulation of the central end of the vagus nerve below the origin 

 of the superior laryngeal branch with moderate induction shocks 

 quickens respiration; strong excitation causes inspiratory spasm and 

 may bring about cessation of breathing in inspiration (Fig. 161). 

 Very weak induction shocks and chemical stimuli e.g., strong KCL 

 solution bring about slowing of respiration or standstill in the 

 expiratory phase (Fig. 162). 



