THE MEDULLA OBLONGATA. 505 



after filling the space inclosed by its convoluted wall, pass through it, 

 and spread out laterally in a divergent direction. 



Physiological Properties of the Medulla Oblongata. The simplest 

 examination of the medulla oblongata shows that its physiological pro- 

 perties are more distinctly marked than those of any other part of the 

 encephalic mass. It is both sensitive and excitable to a high degree, 

 especially in its posterior portions. Artifical irritation, by mechanical 

 or galvanic stimulus, causes at once a painful sensation, provided the 

 rest of the brain be uninjured, and in the recently killed animal produces 

 general convulsive movements of considerable intensity. These effects 

 are due to the irritability of the longitudinal fibres connecting the me- 

 dulla with the spinal cord, and to the roots of the sensitive and motor 

 cranial nerves which take their origin from this part of the encephalic 

 mass. Since the medulla is the only bond of nervous communication 

 between the brain and the spinal cord, its section at any point also 

 destroys voluntary motion and sensibility throughout the body and 

 limbs- 



Action of the Medulla as a Nervous Centre. The various deposits 

 of gray substance in the interior of the medulla, and their connection 

 with nerves of widely different distribution and functions, are the pecu- 

 liar features of its anatomical structure. The results of experiment 

 show that the reflex actions taking place in this part of the nervous 

 system are also of a special and distinctive character. 



The most important of these actions is connected with respiration. 

 So long as the medulla oblongata is left uninjured, although the cranium 

 be emptied of all the other nervous centres, the movements of respira- 

 tion and circulation go on without essential modification. But if the 

 medulla be destroyed, respiration ceases instantaneously. This effect may 

 be produced, without injuring other parts of the brain, in dogs or other 

 warm-blooded animals, by introducing a steel instrument from behind, 

 between the edges of the occipital foramen and the first cervical verte- 

 bra, carrying it forward in the median line until its point rests upon 

 the basilar process of the occipital bone. It is then moved from side to 

 side in such a way as to break up the substance of the medulla oblon- 

 gata, when all the movements of respiration are at once arrested. The 

 circulation continues, and the pulsations of the heart are even increased 

 for a time in force and frequency ; but as the deficiency of aeration in 

 the blood becomes more marked, the circulation is gradually retarded 

 and after several minutes comes to an end. The effect of this operation 

 upon the two functions of circulation and respiration is very different. 

 The circulation is interfered with and finally suspended in a secondary 

 manner, and only because the blood is no longer arterialized ; respira- 

 tion is abolished instantaneously, as the immediate result of the destruc- 

 tion of the medulla oblongata. 



The medulla is, therefore, the most important nervous centre in the 

 brain for the immediate preservation of the vital functions, and the 

 only one whose injury or removal produces at once a fatal result. 

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