Page:Structure and functions of the body; a hand-book of anatomy and physiology for nurses and others desiring a practical knowledge of the subject (IA structurefunctio00fiskrich).pdf/85

 deglutition, mastication, and expression. In the medulla 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 pressure, 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 vertebræ 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 terminale'', 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 enlargements, a cervical enlargement in the lower cervical region, and a lumbar enlargement in the lower dorsal region, 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 ''subarachnoid 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