THE NERVOUS SYSTEM. 79 



deglutition, mastication, and expression. In the med- 

 ulla the nerves that arise in the cerebrum cross over 

 from one side of the body to the other on the crossed 

 pyramidal tracts. The importance of this crossing of 

 the nerve fibers is seen in apoplexy, when a blood-vessel 

 is ruptured in the brain and hemorrhage causes pres- 

 sure, generally on the motor tract. Paralysis of the 

 nerves and of the muscles to which they go results. 

 The paralysis is generally of one side of the body, the 

 opposite side from that on which the injury occurred. 

 The seat of injury in the brain or cord can frequently 

 be determined by the situation and extent of the 

 paralysis. 



Spinal Cord. Extending down from the medulla 

 through the spinal column is the cord. Its length 

 from the foramen magnum, where it begins, down 

 through the vertebrae to the lower border of the first 

 lumbar vertebra, where it ends in a very fine thread- 

 like process with no special function, called the filum 

 terminate, is 17 to 18 inches. Just before it ends a 

 number of nerves are given off in a tail-like expansion 

 known as the cauda equina or horse's tail. It is not 

 uniform throughout its length but presents two enlarge- 

 ments, a cervical enlargement in the lower cervical re- 

 gion, and a lumbar enlargement in the lower dorsal re- 

 gion, where the nerves are given off to the arms and 

 legs respectively. The membranes are the same as 

 those of the brain and are continuous with them, but 

 here the dura mater is not attached to the bony walls 

 enclosing it. For the cord does not fit closely into the 

 canal but is as it were suspended in it. The subarach- 

 noid space communicates with the ventricles of the 

 brain by the foramen of Majendie and is filled with 

 cerebro-spinal fluid for the protection of the cord. In 

 cerebro-spinal meningitis or spotted fever this fluid is 

 infected and for diagnosis lumbar pucture is performed. 



If a cross-section of the cord is made, it is found to 

 have a pretty definite structure. It is roughly cir- 



