442 PHYSIOLOGY 



(the hypoglossal). It sends also efferent fibres to the viscera, which 

 arise from cells in the nucleus ambiguus. These fibres carry motor 

 impulses to the muscles of the larynx and bronchi, and to the 

 oesophagus, stomach, and intestines, secretory fibres to the stomach 

 and inhibitory fibres to the heart. 



At the upper border of the bulb enter also the fibres of the eighth 

 nerve, carrying important impressions from the organ of hearing and 

 the organ of static sense. These will be in all probability divided or 

 injured in isolating the bulb from the higher portions of the brain. 

 While in connection with the upper portions of the brain, the bulb 

 receives also afferent impressions from the skin of the face, and the 

 mucous membrane of the nose and mouth through the descending 

 branches of the root of the fifth nerve, which pass down superficially 

 to the tubercle of Rolando. When in connection with the cord the 

 medulla receives afferent impressions from the whole surface of the 

 body and from all the muscles and joints through the posterior column 

 nuclei. 



The bulbo-spinal animal, i.e. one in whom a section has been 

 carried out at the upper boundary of the medulla, differs from the 

 spinal animal chiefly in the maintenance of the nexus between the 

 visceral functions and the skeleto-motor functions of the body. After 

 removal of all the brain in front of the bulb, the animal still continues 

 to breathe regularly and automatically. The blood pressure and 

 the pulse rate remain normal, and all three mechanisms, respiration, 

 pulse rate, blood pressure, may be affected reflexly by appropriate 

 stimuli, or may be altered in consequence of central stimulation of the 

 medulla. 



In addition to the reflex mechanisms of locomotion, which are 

 evident in the spinal animal, the bulbo-spinal animal shows a greater 

 degree of solidarity in its responses. It is easier to evoke movement 

 of all four limbs. In the frog, if the eighth nerve has been left intact, 

 there is a certain power of equilibration left, and the animal when 

 laid on its back tries to right itself and usually succeeds. 



It is in this portion of the central nervous system that have been 

 located the great majority of the so-called centres. By a statement, 

 that the centre of such-and-such movement or function is situated 

 in the medulla, we mean merely that the integrity of the medulla, or 

 certain parts of it, is essential for the carrying out of the function. 

 Every function, for instance, in which impulses passing up the vagus 

 nerves are involved, is necessarily dependent on the integrity of these 

 nerves and their central connections, and, since these are situated 

 in the medulla, the centres for these functions are also located in this 

 region. From a broad standpoint the medulla or bulb may be looked 

 upon as a ganglion, or a collection of ganglia, whose main office is to 



