134 A MANUAL OF VETERINARY PHYSIOLOGY 



cortex of the brain is connected with the respiratory centre, for 

 the animal may at will increase or withhold its respiration ; 

 while psychical events produce their effects through some such 

 channel, as may be witnessed in the increased respiration of 

 nervous apprehension. A perfectly orderly sequence of events 

 occurs in normal respiration, beginning at the nostrils and end- 

 ing at the flank, the entire smoothness and regularity of which 

 is dependent on the nervous connections of the respiratory centre 

 with the outside. 



The centre is automatic — viz., it is within itself that the dis- 

 charges are generated which issue forth as respiratory impulses ; 

 indeed, it is as automatic in its working as the heart, for if every 

 nervous connection leading to it be divided, the centre continues 

 notwithstanding to act rhythmically. If the chief path through 

 which these impulses gain the exterior be cut — as, for instance, 

 by dividing the spinal cord behind the medulla — death from 

 paralysis of respiration at once ensues. Such a section does not 

 affect the facial muscles of respiration, and an animal that has 

 ceased to breathe and is virtually dead may still continue to make 

 powerful inspiratory facial gasps, the nerves supplying these 

 muscles being derived from a point anterior to the section. 



The nature of the impulses which issue from the respiratory 

 centre depends upon the character- of the impulses received from 

 without, which stimulate their production reflexly. Thus the 

 breathing may be hastened or slowed down, quickened in rhythm 

 and decreased in depth, or both rhythm and depth increased. 

 From the cortex of the brain impulses ma}^ be transmitted, 

 voluntarily increasing the respirations, as in snifhrg, or with- 

 holding them entirety, as when the head is under water. Through 

 the medium of the nasal branch of the fifth pair of nerves im- 

 pulses may be transmitted from the nostrils inhibiting inspira- 

 tion ; from the skin impulses of two kinds are received — viz., 

 those increasing and diminishing respiration. It has even been 

 supposed that different nerve fibres are concerned in the trans- 

 mission of stimulating or inhibiting impulses to the respiratory 

 centre, though not exclusive channels for respiratory purposes. 

 From the larynx through the superior laryngeal nerves impulses 

 inhibiting inspiration and stimulating expiration are transmitted, 

 and the same through the sensory fibres of the glossopharyngeal, 

 which inhibits respiration at the moment of swallowing. In the 

 diagram (Fig. 50) the chief nervous connections of the respira- 

 tory centre are shown ; the sign denotes whether they convey 

 impulses which stimulate or inhibit the respiratory centre. 



The Influence of the Vagus on Respiration . — The vagus is the 

 most important afferent channel by which the lungs are brought 

 into connection with the medulla ; the sensory fibres cover the 



