Elementary Biology. 



the broad triangular-like area of the myelencephalon, or 

 medulla. 



The medulla is continued directly into the spinal cord, 

 or myelon. It is relatively of considerable size, and for 

 the greater part of its length is a thick broad belt, with a 

 well marked groove running down its upper or dorsal sur- 

 face. Its latter third is much thinner and narrower than 

 the rest and terminates in a delicate thread, the filum 

 terminale, which lies within the anterior end of the urostyle. 



The cranial nerves. There are ten pairs of cranial 

 nerves, all save the first springing from the ventral aspect 

 of the brain, and, after escaping from the cranium, breaking 

 up into finer branches and becoming distributed to the 

 various organs which it is their duty to supply. They 

 may be tabulated as follows : 



From the spinal cord also ten pairs of spinal nerves 

 are given off. Each nerve arises from the cord by two roots, 

 one dorsal or posterior, the other ventral or anterior. The 

 two roots unite before leaving the neural canal, and the 

 nerve formed by their union passes through one of the 

 intervertebral foramina, as the spaces between the several 

 vertebrae are termed. The posterior root, which is known 

 as the sensory root, for a reason to be afterwards explained 



