RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 279 



even after the circulation has been stopped altogether by excision 

 of the heart, quiet, regular breathing may be seen for a considerable 

 time. Of course, blood is essential for the continued nutrition of the 

 centre and its connections, and it eventually breaks down and ceases 

 to discharge. The respiratory discharge is still less dependent for its 

 initiation upon the arrival of afferent impulses. For after section 

 of the bulb above the centre, of the cord below the origin of th^. 

 phrenics, of the vagi and of the posterior roots of all the upper cer- 

 vical nerves, the spasmodic respiration which we have already 

 described as occurring when the vagi and the higher paths have been 

 severed continues without essential modification. It has also been 

 observed that during resuscitation of the bulb and upper cervical 

 cord after a period of anaemia, stimulation of afferent nerves, in- 

 cluding the vagi, is entirely without influence on the respiratory 

 movements for some time after respiration has returned, presumably 

 because the synapses (p. 824) on the afferent paths lying within the 

 previously anaemic area are as yet unable to conduct the nerve 

 impulses. Nevertheless, the respiratory centre continues steadily 

 to discharge itself along the efferent paths, whose synapses are 

 situated beyond the anaemic region. Section of the bulb above 

 the level of the respiratory centre, and of the cord below the origin 

 of the phrenic nerves, in addition to the anaemia, makes no essential 

 difference in the result. The initial rate of discharge of the centre 

 thus isolated from afferent impulses is approximately constant in 

 different experiments (about four a minute in cats). 



Spinal Respiratory Centres. Although the chief respiratory centre 

 lies in the medulla oblongata, under certain conditions impulses to 

 the respiratory muscles may originate in the spinal cord. Thus, in 

 young mammals (kittens, puppies), especially when the excitability 

 of the cord has been increased by strychnine, in birds and in alli- 

 gators, movements, apparently respiratory, have been seen after 

 destruction of the brain and spinal bulb. In adult cats, when 

 the functions of the brain, medulla, and cervical cord have been 

 abolished by occlusion of their vessels, similar movements of the 

 thoracic and abdominal muscles may be seen, but they are not suffi- 

 cient for effective respiration. No proof has ever been given that 

 in the intact organism the spinal cord below the level of the bulb 

 takes any other part in respiration than that of a mere conductor of 

 nerve impulses; and it is not justifiable to assume the existence of 

 automatic spinal respiratory centres on the strength of such experi- 

 ments as these. 



Death after Double Vagotomy. Alterations in the rhythm of respira- 

 tion are not the only effects that follow division of both vagi (or vago- 

 sympathetics) in the neck. In certain animals, at least, this operation 

 is incompatible with life. In the rabbit, as a rule, death takes place in 

 twenty-four hours. A sheep may live three days, and a horse five or 

 six. Dogs often live a week, occasionally a month or even two, and in 



