THE MEDULLA SPINALIS. 559 



occupy exactly the centre of the canaj, but, according to Olli- 

 vier, rests against its anterior wall in the vertical position, and 

 inclines towards the posterior, especially at its lower part, wjsen 

 the individual rests upon his back. The consistence of the 

 medulla is much greater when healthy and observed in the recent 

 state, than is generally supposed ; it surpasses that of the cere- 

 brum and cerebellum, though it is in general less than that of 

 the pons varolii. 



The weight of the medulla spinalis, separated from its mem- 

 branes and nerves, is in the adult in proportion to the entire 

 mass of the brain, according to Meckel, as one to forty. Man, 

 however, is, of all animals, the one in which the medulla spi- 

 nalis is the least in comparison with the brain : a fact, well 

 exemplified by comparative anatomy, in which it will be found 

 that the more we recede from man in the scale, the greater is 

 the proportion of the medulla to the encephalon. The spinal 

 medulla presents in its course two remarkable enlargements ; 

 one, superior, named cervical or brachial, which extends from 

 the third cervical vertebra to the second dorsal ; and the second, 

 between the fourth dorsal vertebra and the second lumbar, 

 which is called the lumbar or crural. Below the crural plexus 

 the medulla is fusiform, and terminates most generally in a 

 point, but sometimes in a bulb, and occasionally in a bifurca- 

 tion. 



The spinal cord, is divided into two symmetrical portions 

 that is, one for each side of the body by two median fissures ; 

 one called the anterior median fissure, forms a groove on its front 

 surface of a depth equal to one-third of the diameter of the cord, 

 the other groove on its back part, called the posterior median 

 fissure, is deep at the upper part of the medulla, but becomes 

 quite superficial below. The anterior median fissure -is more 

 distinct and wider than the posterior. Both fissures run the 

 whole length of the cord, and are to be found at all periods of 

 life. In fig. 219, are seen transverse cuts of the medulla, show- 

 ing its division by the anterior and posterior median fissures, into 

 the two symmetrical halves. 



