FUNCTIONS OF THE MEDULLA OBLONGATA. 407 



centripetal impression to excite motion may be conducted, and 

 the number and distance of those through which the motor 

 impulse may be directed. The principal centripetal nerves 

 engaged in respiration are the pneumogastic, whose branches 

 supplying the lungs appear to convey the most acute impres- 

 sion of the " necessity of breathing." When they are both 

 divided, the respiration becomes slower (J. Reid), as if the 

 necessity were less acutely felt : but it does not cease, and 

 therefore other nerves besides them must have the power of 

 conducting the like impression. The experiments of Volk- 

 mann make it probable that all centripetal nerves possess it 

 in some degree, and that the existence of imperfectly aerated 

 blood in contact with any of them acts as a stimulus, which, 

 being conveyed to the medulla oblongata, is reflected to the 

 nerves of the respiratory muscles : so that respiratory move- 

 ments do not wholly cease so long as any centripetal nerves, 

 and any nerve supplying muscles of respiration, are both in 

 continuous connection with the respiratory centre of the 

 medulla oblongata. The circulation of imperfectly aerated 

 blood in the medulla oblongata itself may also act as a stimu- 

 lus, and react through this nerve-centre on the nerves which 

 supply the inspiratory muscles. 



The wide extent of connection which belongs to the medulla 

 oblongata as the centre of the respiratory movements, is further 

 shown by the fact that impressions by mechanical and other 

 ordinary stimuli, made on many parts of the external or inter- 

 nal surface of the body, may induce respiratory movements. 

 Thus involuntary respirations are induced by the sudden con- 

 tact of cold with any part of the skin, as in dashing cold water 

 into the face. Irritation of the mucous membrane of the nose 

 produces sneezing. Irritation in the pharynx, oesophagus, stom- 

 ach, or intestines, excites the concurrence of the respiratory 

 movements to produce vomiting. Violent irritation in the rec- 

 tum, bladder, or uterus, gives rise to a concurrent action of the 

 respiratory muscles, so as to effect the expulsion of the faeces, 

 urine, or foetus. 



The medulla oblongata appears to be 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 

 (see p. 213). 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 marsupial 

 embryos before the brain is developed ; and by the complete 

 arrest of the power of swallowing when the medulla oblongata 



