404 PHYSIOLOGY 



each tract is very conflicting. The following account represents 

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

 (Fig. 177) : 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 ascending tract of Gowers, 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 muscular 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 hyperaethesia 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 hyperaesthetic parts, and 

 indicating the level of the lesion in the cord. This anaesthesia is due 

 to the fact that the conductors 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 on one side, pass down that side till they reach the 

 lower part of the medulla. Here the greater number of the fibres 



