440 THE NERVOUS SYSTEM. 



appearance of its longitudinal tracts, and especially by the changed 

 position and special properties of its gray substance. 



The arrangement of the gray substance is one of the most character- 

 istic features of the medulla oblongata. First, it increases in quantity 

 from below upward ; and, secondly, it undergoes a complete alteration 

 in form and position. In the spinal cord it presents the well-known 

 figure, on transverse section, of a central mass extending on each side 

 into the anterior and posterior horns. But in the medulla oblongata 

 it recedes into a backward position ; its posterior horns spreading out 

 laterally, and the remainder occupying the space between them. The 

 Dosterior median fissure also becomes shallower and wider by the diver- 

 gence of the posterior columns ; and the central canal approximates the 

 posterior wall of the medulla, finally opening upon its surface at the 

 lower part of the fourth ventricle. The gray substance of the medulla 

 is thus uncovered posteriorly, forming a superficial layer on each side 

 the median line, immediately beneath the floor of the fourth ventricle. 

 It thence extends forward, without complete interruption, through the 

 whole length of the fourth ventricle and about the aqueduct of Sylvius ; 

 giving origin, at various points in this situation, to the root-fibres of 

 all the cranial nerves, excepting the olfactory and the optic. 



Physiological Properties of the Medulla Oblongata. The physio- 

 logical properties of the medulla are more distinctly marked than those 

 of any other part of the encephalic mass. It is in a high degree both 

 sensitive and excitable, especially in its posterior portions. Either 

 mechanical or galvanic irritation gives rise at once to signs of sensa- 

 tion, if the rest of the brain be uninjured, and in the recently killed ani- 

 mal produces convulsive movements of considerable intensity. These 

 effects are due to irritation of the longitudinal fibres connecting the 

 medulla with the spinal cord, and of the sensitive and motor cranial 

 nerve roots. Since the medulla is the only bond of nervous communi- 

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

 also destroys voluntary motion and sensibility in the body and limbs. 



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

 deposits of gray substance in the medulla, and their connection with 

 nerves of widely different distribution and functions, are the peculiar 

 features of its anatomical structure ; while its reflex actions are also of 

 a special and distinctive character. 



The most important action of the medulla as a nervous centre is that 

 connected with respiration. So long as the medulla is uninjured, al- 

 though the cranium be emptied of all its other nervous centres, respi- 

 ration goes on without essential modification. But if the other parts 

 of the brain be left intact and the medulla be destroyed, in any warm- 

 blooded animal, all movements of respiration cease instantaneously. 

 The circulation still continues for a time ; but as the blood becomes 

 deficient in aeration, it 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 



