THE SPINAL CORD AS A CONDUCTOR 



one segment of the cord to another through grey and white matter alter- 

 nately. It is supposed that all of the ascending tracts may convey afferent 

 impulses from the posterior spinal roots to the brain, although evidence as to 

 the part taken by each tract is very conflicting. The following account 

 represents the views which may be regarded as the most probable (Page May) 

 (Fig. 176) : Pain impulses, on entering the cord by the posterior roots, cross 

 to the other side at once, and then pass up, chiefly in the antero-lateral column, 

 by the spino-thalamic fibre s, as far as the optic thalamus. Sensations of heat 

 and cold take a very similar course. Hence they are generally affected by 

 lesions of the cord in the same way as pain sensations. Impulses of touch 

 and pressure, after entering the cord, pass up in the posterior column of 

 the same side for four or five segments, then cross gradually and pass up in the 

 opposite anterior column. Impulses serving muscular sensibility, including 

 the impulses from joints and tendons, take two courses. Those which do not 

 reach consciousness, and are involved in the involuntary guidance of mus- 

 cular movements, run up chiefly in the anterior and posterior cerebellar 

 tracts of the same side. Those which furnish the material for conscious 

 sensations and give information as to the position of the limbs, &c., are 

 entirely homolateral, and travel up in the posterior columns of the same side 

 of the cord. All impulses which reach the brain cross finally to the optic 

 thalamus and thence to the cerebral cortex of the opposite side. 



Hemisection of the cord on one side, as was first pointed out by Brown 

 Sequard, causes the following symptoms : 



(1) Paralysis of the voluntary motor conductors on the same side. 



(2) A paralysis also of the vaso-motor conductors on the same side, and, 

 as a consequence, a greater afflux of blood, and a higher temperature. There 

 may be some degree of hyperaesthesia on this side. 



(3) There is anaesthesia affecting all kinds of sensibility, excepting the 

 muscular sense, in the opposite side to that of the lesion, owing to the fact 

 that the conductors of sensitive impressions from the trunk and limbs 

 decussate in the spinal cord ; so that an injury in the cervical region of that 

 organ in the right side, for instance, alters or destroys the conductors from 

 the left side of the body. 



(4) There is some degree of anaesthesia also on the side of the lesion, in 

 a very limited zone, above the hypera3sthetic parts, and indicating the level 

 of the lesion in the cord. This anaesthesia is due to the fact that the con- 

 ductors of sensory impressions, reaching the cord through the posterior roots, 

 at the level or a little below the seat of the alteration, have to pass through 

 the altered part to reach the other side of the cord. 



The only direct unbroken cortico-spinal fibres are those contained in the 

 pyramidal tracts. Motor impulses, which start from the cerebral cortex 

 one side, pass down that side till they reach the lower part of the medulla 

 Here the greater number of the fibres cross over in the pyramidal deci 

 to run down in the crossed pyramidal tract on the other side of the c 

 The few fibres which do not cross over in the pyramidal decussation a 

 continued as the direct or anterior pyramidal tract. These, however, als< 



