CENTRES IN THE MEDULLA OBLONGATA. 887 



apparatus is cut oft' from the cord. The fall of arterial blood pressure, 

 consequent on the interference with the bulbar vasomotor centre, is not 

 responsible for the difference in the reflexes. 



Ablation of the rest of the bulb annuls the respiratory movements, 

 depresses the arterial blood pressure, and destroys a number of vaso- 

 motor reflexes that were previously possible. The cardio-inhibitory 

 centre connected with the vagus and its accessory nerve is also de- 

 stroyed, and the reactions through it of sensory nerves upon the heart 

 are of course lost with it. 



In the bulb lie also, in major part, the nervous mechanisms guarding 

 and working the apparatus of the great anterior opening of the body, by 

 which material from the environment is seized, tested, and admitted 

 as pabulum for digestion. The respiratory apparatus belongs to this 

 same system, although with additional excretory functions, and in higher 

 groups partly subservient to emotional and mental expression. Nothing 

 perhaps shows more clearly the want of functional solidarity of the 

 individual " segments," as morphologically defined, than the profound 

 degree to which the nervous " centre " for the ingestion of food extends 

 across the boundary between metencephalon and myelencephalon in the 

 floor of the fourth ventricle. This " centre " pervades both, and in its 

 spatial distribution, as in that of the bulbar mechanism of speech, little 

 can be traced of any boundary between them, although its fractions can 

 be located more or less independently. These are often referred to as 

 " centres " of deglutition, phonation, vomiting, coughing, hiccough, etc. 

 With all its complexity, the mechanism is essentially that of a great 

 oral sphincter. Special details are given concerning them in various 

 appropriate sections of this work. The visceral sensory surfaces and 

 alimentary epithelium connected with the myelencephalon extend 

 far backward. The reflex centres for movement of the oesophagus, and 

 for secretion by the gastric l and pancreatic glands, 2 lie in the caudal 

 part of the bulb, in connection with the vagus and its accessory nerve. 

 After removal of the entire encephalon in the frog, the intestines enter 

 into a condition of long-continued peristaltic activity. If, however, the 

 bulb be left, a status of apparently abnormal quietude obtains in the 

 intestine. 3 A point of general interest in the physiology of the great 

 "alimentary" nerve centre in the bulb is the high degree to which 

 it employs inhibition. Each subdivision of it is depressible by " in- 

 hibitory " fibres from some afferent nerve trunk or another ; thus, the 

 respiratory portion by fibres in the superior laryngeal nerve, the 

 deglutition portion of it by fibres in the superior laryngeal, and partly 

 in the inferior laryngeal, the secretory portion by fibres in the afferent 

 branches of the gastric and thoracic vagus, and so on. 4 



The term centre has been used with perhaps especial laxity in speaking of 

 the bulb. A "convulsion" centre has been, among other centres, postulated 

 for the bulb. Widespread convulsions of the skeletal musculature can be 

 produced by chemical stimulation there. 5 A drop of 10 per cent, sodium- 



1 Pawlow, Arch. f. Physiol., Leipzig, 1895 (witli Schowmow-Simanowski). 

 - Pawlow, ibid., 1893, Suppl. (in Russian, 1888). 



3 Steinach, Arch, f. d. ges. Physiol, Bonn, 1895, Bd. lx., S. 593 ; 1898, Bd. lxxi. S. 523. 



4 See especially Kronecker & Meltzer, Arch. f. Physiol., Leipzig, 1881, S. 465; ibid., 

 1883, Suppl. S. 328 ; also Monatsbl. d. Berl. Ahad. d. 'll'issnuxh., 1881 ; also later papers 

 from Kronecker, Bern Laboratory, and S.J. Meltzer, Trans. New York Acad. Med., Jan. 1899. 



5 Heubel, Arch. f. d. yes. Physiol., Bonn, 1874, Bd. ix. ; Lapinsky, ibid., 1899, Bd. 

 lxxiv. S. 47. 



