330 NERVOUS SYSTEM. 



mity or the point of the medulla spinalis. The posterior chord, 

 though so much smaller and narrower than the anterior, is it- 

 self subdivided into two, by a slight but well marked split; of 

 these two last chords, the one next to the posterior middle fissure 

 of the medulla is smaller than the other. These arrangements, 

 according to Meckel, are much more obvious in the early life 

 of the human subject, than afterwards, arid are particularly con- 

 spicuous in the brute creation. 



The thin white lamina? by which the two sides of the spinal 

 marrow adhere to each other at the bottom of the middle fis- 

 sures, are called, by modern anatomists, Anterior and Posterior 

 Commissures. Their precise arrangement is not yet fully ascer- 

 tained, but it is stated by Gall and Spurzheim,* that the Anteri- 

 or Commissure is formed by transverse fibres or filaments, which 

 adhere to one another from the opposite sides like a suture, or 

 after a serrated fashion; whereas, the Posterior Commissure is 

 formed by a band of longitudinal fibres. There is also another 

 Commissure, called Middle or Cortical, from its position, and 

 from its being formed out of the transverse part of the grayish 

 or cineritious substance. 



The chords which form each half of the medulla are differ- 

 ently disposed : the posterior continues on the side to which it 

 specially belongs, while the anterior having got within the cir- 

 cumfert nee of the first cervical vertebra, crosses over to the op- 

 posite side by decussating with its fellow. This deeussation oc- 

 cupies the space of four or five lines, and interrupts, for that 

 distance, the middle fissure in front of the medulla. It is not 

 effected by the chords passing in mass from one side to the other, 

 but each chord sends off four or five fasiculi, which are inter- 

 woven with their congeners, like the fingers of the two hands 

 when interlocked obliquely. It is to be observed that the whole 

 mass of the anterior chords is not subjected to such distribution; 

 for the fasciculi just described come from their anterior and 

 from their posterior faces, while the intermediate part is per- 

 mitted to pursue its course straight upwards. This deeussation, 

 upon which so much interesting physiological speculation de- 

 pends, though known for the last century, and spoken of by 

 Mistichelli and Petit, has been strangely overlooked by many 



* Recherches sur le Syst. Nerv. ct sur cclui du Cerveau. Paris, 1809. 



