THE NERVOUS SYSTEM. 595 



hemispheres and cerebellum; its existence in anencephalous monsters ; 

 the power of swallowing possessed by the marsupial onibryo 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.) Bilateral centres 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 divi- 

 sion of the oth for the muscles of the jaw. 



(4.) Bilateral centres for the Secretion of Saliva, which have been 

 already mentioned (p. 333). 



(5.) Bilateral centres for Vomiting (p. 369). 



(6.) Bilateral centres for Coughing, which are said to be independent 

 of the respiratory centre, being situated above the inspiratory part of 

 that centre. 



(7.) Bilateral centres for Sneezing, connected no doubt with the 

 respiratory centre. 



(8.) Bilateral centres 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 centres. 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.) Car dio- Inhibitory centres. The action of these centre in main- 

 taining the proper rhythm of the heart through the vagus fibres, which 

 terminate in a local intrinsic mechanism, has been already discussed. 

 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 stimulat- 

 ing any sensory nerve, including the vagus itself. 



(3.) Accelerator centres for the heart. The centres from which arise 

 the accelerator fibres of the heart, in the medulla. They are automatic 

 but not tonic in action. 



(4.) Vaso-motor centres, which control the unstriped muscle of the 

 arteries, are also situated in the medulla. Like the respiratory centre, 



