PRACTICAL CONSIDERATIONS : THE SPINE. 145 



appear early, and are chiefly due to the shape of the vertebral bodies. The cervical 

 and lumbar curves are secondary, develop after birth, and depend mainly on the 

 shape of the disks. Greater mobility would naturally be expected under the latter 

 circumstances. The close articulation between the separate vertebrae throughout 

 the whole column, while it renders a slight degree of sprain not uncommon, tends 

 at the same time to diffuse forces applied to the spine and to concentrate them within 

 certain areas. These areas are the points at which fixed and movable portions of the 

 spine join each other, as in the neighborhood of the atlanto-axial, the cervico- 

 thoracic, and the thoracico-lumbar regions. 



If the force is sufficient to cause an injury of greater severity than a sprain 

 it is apt to be a dislocation or a fracture with dislocation at one or other of these 

 localities. The latter accident is usually caused by extreme flexion of the spine, and 

 of the three points mentioned is most often found in the segment including the 

 lower two thoracic and th^ upper one or two lumbar vertebrse. This is due to the 

 fact that ( I ) this segment has to bear almost as much weight as the lumbar spine, 

 and yet its vertebrae are smaller and weaker. (2) The transverse processes are 

 short, while the longer ones below, together with the crest of the ilium and the ribs 

 abpve, give a powerful leverage to the muscles that move the region in question. 

 (3) It is the region at which the most concave part of the thoracico-lumbar curve is 

 found, making the "hollow of the back" and corresponding to the "waist" where 

 the circumference of the trunk is smallest. (4) Its nearness to the middle of the 

 column enables a greater length of leverage to be brought to bear against it than 

 against any other part. (5) The different segments of the spine above it are com- 

 paratively fixed (Humphry). These anatomical facts account for the frequency 

 and severity of the injury known as " fracture-dislocation" in this region as a result 

 of extreme flexion. 



A view of the vertebral column from behind (Fig. 170) serves well to illustrate 

 some of these points. 



Pure dislocations are rare, but are more frequent in the upper than in the lower 

 part of the spine, because the bodies of the cervical vertebrae are small, and the 

 interlocking of the articular processes is less firm than it is lower in the column. 

 The vertebra most commonly dislocated is the fifth cervical, which might be expected 

 from the fact that in the neck flexion and extension are freest between the third and 

 sixth vertebrae. The dislocation is usually anterior, — that is, the articular process 

 of one vertebra slips forward and falls down on the pedicle of the vertebra below, 

 resting in the intervertebral notch, — this accident being rendered easy by the com- 

 paratively horizontal position of the articular processes in the cervical region. Such 

 dislocation is practically impossible in the thoracic or lumbar region without fracture, 

 while fracture is comparatively rare in the cervical region. The lumen of the spinal 

 canal may be but little, if at all, invaded. 



As to reduction, experiments show (Walton) that no moderate amount of exten- 

 sion in a direct line would raise the displaced articular processes in the least degree. 

 It was, however, found easy to unlock these processes by retro-lateral flexion, bend- 

 ing the head towards the side to which the face was already turned, an inappreciable 

 amount of force being necessary. Rotation into place completed the reduction. 



All pure dislocations are really subluxations, as without extensive fracture of 

 the processes and great laceration of ligaments a complete separation of the articu- 

 lar surfaces of two adjoining vertebrae is practically impossible. 



Pure fracture, not the result of a gunshot wound, is rare. If from flexion, the 

 fracture involves the body ; if from direct violence, usually the laminae. These facts 

 require no explanation. 



Dislocations and fractures of the upper two cervical vertebrae are especially 

 serious on account of the proximity of the medulla and of their position above the roots 

 of the phrenic nerve and of the nerves supplying the external muscles of respiration. 

 If the accident is from overflexion, it may be a dislocation between the occiput and 

 the atlas, as it is there that the movements of flexion and extension of the head take 

 place. If it arises from extreme rotation, and especially if there is rupture of the 

 check ligaments, it may be a dislocation of the atlas from the axis, as it is there that 

 the rotary movements of the head occur. " A dumb person expresses ' yes' at the 



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