PRACTICAL CONSIDERATIONS : THE BRAIN. 1217 



on the same side. We have already seen that lesions of the cortex produce mono- 

 plegia, unless large enough to involve the whole motor zone, but cortical hemiplegia 

 is much more common than cortical monoplegia. In the internal capsule the motor 

 fibres are gathered together so compactly that a small lesion, as an apoplectic hemor- 

 rhage, will frequently interrupt the whole tract and give a hemiplegia of the opposite 

 side of the body. 



In the medulla and cord the tracts of both sides are so close together that a 

 lesion may easily paralyze both sides (paraplegia) ; indeed, diseases of the cord fre- 

 quently involve the whole transverse section, paralyzing sensation as well as motion. 



' ' Regarding disturbances of sensation, it is of importance to recall the anatomical 

 relations of the chief sensory paths, the mesial fillet and the spino-thalamic tract. 

 The former arises from cells within the gracile and cuneate nuclei of the medulla, 

 around which cells the long fibres of the posterior column end. The fibres of the 

 fillet-tract, therefore, cross in the sensory decussation within the medulla. The fibres 

 of the spino-thalamic tract, on the other hand, are axones of spinal cells situated on 

 the opposite side and undergo crossing within the cord. Within the brain-stem, the 

 two paths are closely associated and lesions within the medulla may involve both sets 

 of fibres, leading to complete hemi-anesthesia of the opposite side. Unilateral lesions 

 of the cord, on the contrary, produce only partial hemi-anesthesia, since within the 

 cord the tracts ascend on different sides. ' ' 



Hemiplegia is, therefore, the common form of cerebral paralysis ; paraplegia the 

 common form of spinal paralysis ; while 77io7ioplegia occasionally results from lesions 

 of the brain cortex, but more commonly from lesions of peripheral nerves. 



The sides and convexity of the brain can be exposed for operation, so that lesions 

 of the cortex can be attacked and often removed ; but the region of the internal 

 capsule, which is near the basal ganglia, cannot be reached. 



The soft brain may be injured by contact with its bony walls when the head is 

 violently shaken, the spaces surrounding the brain and filled with fluid permitting 

 considerable movement of the brain. The injury in cerebral contusion occurs more 

 frequently on the under surface, both as regards the cerebrum and cerebellum, than 

 on any other part (Prescott Hewett). That portion, however, which includes the 

 medulla, pons, and interpeduncular space, rests on a large collection of cerebro- 

 spinal fluid, and is least frequently injured. 



