ACCORDING to Markwald [1887, 1890], when bilateral division of the vagi in a rabbit is preceded or followed by transection of the brain stem immediately behind the posterior colliculi, the respiration is profoundly altered, it ceases to be rhythmical, and consists of an irregular series of long powerful inspiratory " cramps ", each of several seconds' or minutes' duration and only interrupted at irregular intervals by short expiratory pauses. If the nucleus of the trigeminus nerve is intact and functional, the inspiratory cramps begin gradually to become shorter, eventually being reduced to powerful, cramp-like respiratory acts of a few seconds' duration, following one after the other in a more or less regular, rhythmical series. If, however, the nucleus of the trigeminus nerve has been damaged, as shown by the disappearance of the corneal and nose reflexes, the inspiratory cramps remain long and irregular and lead to death from asphyxia. When division of the vagi, on the other hand, is performed in an animal decerebrated just in front of the posterior colliculi, or at any higher level, the effects on respiration are not different from those which are usually observed after vagotomy in an animal with the central nervous system intact: i.e. they simply consist in a deepening and slowing of the breathing. Markwald concluded that in the posterior colliculi there is a centre inhibitory to inspiration, whose action becomes clear when the inhibitory activity of the vagi is abolished by their section. He further suggested that the nucleus of the trigeminus nerve, when both the vagi and the posterior colliculi had been put out of action, would acquire a new tonus and exercise a rhythmical inhibitory influence, though a less efficient one, upon the inspiratory centre.
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