THE CEKEBKO-SPINAL NEKVOUS SYSTEM. . 497 



(2.) A centre for the movements of Deglutition. The medulla ob- 

 longata appears to contain the centre whence are derived the motor 

 impulses enabling the muscles of the palate, pharynx, and oesophagus to 

 produce the successive co-ordinate and adapted movements necessary to 

 the act of deglutition (p. 245). This is proved by the persistence of 

 swallowing in some of the lower animals after destruction of the cerebral 

 hemispheres and cerebellum; its existence in anencephalous monsters; 

 the power of swallowing possessed by the marsupial embryo before the 

 brain is developed; and by the complete arrest of the power of swallow- 

 ing when the medulla oblongata is injured in experiments. 



(3.) A centre for the combined muscular movements of Sucking, the 

 motor nerves concerned being the facial for the lips and mouth, the 

 hypoglossal for the tongue, and the inferior maxillary division of the 5th 

 for the muscles of the jaw. 



(4.) A centre for the Secretion of Saliva, which has been already 

 mentioned (p. 237). 



(5.) A centre for Vomiting (p. 258). 



(6.) A centre for Coughing, which is said to be independent of the 

 respiratory centre, being situated above the inspiratory part of that 

 centre. 



(7.) A centre for Sneezing, connected no doubt with the respiratory 



centre. 



(8.) A centre for the Dilatation of the pupil, the fibres from which 

 pass out partly in the third nerve and partly through the spinal cord 

 (through the last two cervical and two upper dorsal nerves?) into the 

 cervical sympathetic. 



(b ) Automatic centres. 



(1.) Respiratory centre. The action of the respiratory centre has 

 been already discussed. It is only necessary to repeat here that although 

 it can be influenced by afferent impulses, it is also automatic in its 

 action, being capable of direct stimulation, as by the condition of the 

 blood circulating within it. It is also bilateral. It probably consists of 

 an inspiratory part and of an expiratory part. The centre is capable of 

 being influenced both reflexly and to a certain extent also by voluntary 

 impulses. The vagus influence is probably constant in the direction of 

 stimulating the inspiratory portion of the centre, whereas the influence 

 of the superior laryngeal is not always in action, and is inhibitory. 



(2 ) The Cardio-Inhibitory centre. The action of this centre in 

 maintaining the proper rhythm of the heart through the vagus fibres, 

 which terminate in a local intrinsic mechanism, has been already dis- 

 cussed. The centre can be directly stimulated, as by the condition of 

 the blood circulating within it, and also indirectly by afferent stimuli, 

 especially by stimulating the abdominal sympathetic nerves, but also by 

 stimulating any sensory nerve, including the vagus itself. 

 32 



