I052 HUMAN ANATOMY. 



The bony canal is lined with periosteum, unlike the cranium, in which the 

 external layer of the dura mater serves that purpose. The spinal dura is separated 

 from the posterior common ligament, the ligamenta subflava, and the periosteum by a 

 fatty areolar tissue containing a plexus of veins. Extensive extradural hemorrhage 

 may, therefore, occur without serious pressure on the cord. The blood tends to sink 

 by gravity, and later may produce symptoms of compression. The dura is thick 

 and strong and offers considerable resistance to the invasion of disease from with- 

 out, even to tuberculosis with caries of the vertebrae, or to malignant tumors arising 

 within the vertebrae. Infections outside the spinal column, as in abscess of the back, 

 or bed sores, may extend along the communicating veins, giving rise to extradural 

 abscess and perhaps to extensive meningitis. 



The spinal cord, surrounded by cerebro-spinal fluid, hangs loosely within the 

 dura, being attached to it only by the roots of the spinal nerves which receive invest- 

 ments from the dura as they pass outward, by the ligamenta denticulata, and by the 

 delicate fibres of arachnoid tissue extending from the pia to the dura. The cord is, 

 therefore, not frequently injured from external violence. The numerous articulations 

 of the vertebrae and the elasticity of the ligaments and of the intervertebral discs permit 

 the distribution of much of the force applied to the spine before it reaches the cord. 



The greater part of the cerebro-spinal fluid is contained in the subarachnoid 

 space, which communicates freely with the same space in the cranium, and is con- 

 tinuous with the ventricular fluid through the foramen of Majendie. 



The cord is exposed to the danger of penetration by sharp instruments only from 

 behind, but even here the overlapping of the laminae and spinous processes offers 

 an excellent protection. This protection is largely lacking above and below the atlas, 

 and the risk there from siich wounds is correspondingly greater. At lower levels in 

 order that the canal may be reached, the vulnerating instrument must be directed in 

 the line of the obliquity of the laminae, which will vary in the different portions of 

 the spine, being greatest in the dorsal region. 



Concussio7i — shaking with molecular disturbance and without obvious gross 

 lesion — of the cord, although more frequent than has been supposed, is rare because 

 of (a) the arrangement of the different constituents of the vertebral column, which by 

 means of its curves, the elastic intervertebral discs, its numerous joints, and the 

 large amount of cancellous tissue in the vertebral bodies, is able to take up and 

 distribute harmlessly forces of some degree of violence ; (^b) the situation of the cord 

 in the centre of the column, where, as the most frequent serious injuries to the spine 

 are caused by extreme forward flexion, it is somewhat removed from danger in 

 accordance with a law of mechanics that ' ' when a beam, as of timber, is exposed to 

 breakage and the force does not exceed the limits of the strength of the material, 

 one division resists compression, another laceration of the particles, while the third, 

 between the two, is in a negative condition" (Jacobson) ; {c) the suspension of the 

 cord in the surrounding cerebro-spinal fluid ( ' ' like a caterpillar hung by a thread 

 in a phial of water" — Treves) by its thecal attachments and nerve-roots ; (y) its 

 connection above with the cerebellum, itself resting on an elastic "water-bed" 

 which minimizes the transmission downward of violence applied to the cranium. 

 Many of the cases reported as concussion are undoubtedly due to hemorrhage or 

 other gross lesions of the cord. 



Contusion of the cord may occur from sprains, as in forced flexion of the spine. 

 The most frequent and most serious cases are those due to fracture-dislocations of the 

 spine, the cord being more or less crushed between the upper and lower fragments. 

 It is so delicate a structure that it may be thoroughly disorganized without evident 

 injury to the membranes or alteration of its internal form. The paralysis of the parts 

 below will be complete or partial according to whether the whole or only a part of 

 the transverse section of the cord at the seat of injury is destroyed. Since when 

 the lesion is complete everything supplied by the cord below the seat of the lesion 

 is paralyzed, the higher the injury to the cord the greater the gravity of the case. 

 When the atlas or axis is fractured and displaced the vital centres in the medulla are 

 in danger and death may result immediately. The phrenic nerves which arise chiefly 

 from the fourth cervical segment, but partly from the third and fifth segments, 

 are also paralyzed and respiration ceases. 



