CHAPTER XL 



THE SPINAL MARROW AND ITS FUNCTIONS. 



1. WE have seen, that of all animals, the brain of man 

 has the greatest development, in proportion to the spinal 

 marrow and nerves ; if we descend to the lower animals, 

 such as reptiles and fishes, we shall find that the brain^pre- 

 sents merely a delicate, anterior termination of the spinal 

 cord, not much larger in diameter than the cord itself./ In 

 these animals, we find the instincts very limited, and the 

 displays of sagacity extremely obscure ; while they possess 

 great tenacity of life. Frogs, for example, continue to jump 

 about for many hours after their heart has been torn out ; 

 and the heart of the shark will palpitate as long after it has 

 been removed from the body. Turtles have been known to 

 live for months after the whole brain has been scooped out. 

 These facts prove, that such an imperfect development of 

 the cerebral mass, leaves the different systems of organs less 

 intimately connected and more independent of each other. 



2. If we descend still lower in the scale of invertebrate 

 animals, we shall find the different parts still more independ- 

 ent of each other ; for if we divide their bodies, so that each 

 part will possess a nervous ganglion, it will become a separ- 

 ate individual, and exercise all the functions performed by 

 the entire animal ; and if we descend to zoophytes, we find 

 that in cutting them into threads, however minute, each 

 piece will retain life and continue to grow. 



3. Medulla oblongata. -J^here is no distinct line of demar- 

 cation between the medulla oblongata and the spinal corcf; 

 For this reason, they are considered and described as one 

 organ. The medulla oblongata, however, consisting of six 

 columns, gives rise to several nerves of distinct function, and 



