140 INTRODUCTION TO NEUROLOGY 



over the whole of the opposite side of the body below the level 

 of the injury, but there would be no disturbance of either tem- 

 perature or pain sensibility. Similarly, by an injury of the trac- 

 tus spino-thalamicus lateralis, pain or temperature sensibility 

 might be lost with no disturbance of pressure sense. (For the 

 description of a case of this sort see p. 173.) 



Such combinations of symptoms as just described could 

 not occur from any form of injury to the peripheral nerves, for 

 in these nerves the various kinds of fibers are all mingled in the 

 larger trunks, so that one functional component cannot be in- 

 jured without involvement of the others also. And at the first 

 division of these trunks into deep and superficial branches each 

 branch also carries all or nearly all of the functional systems 

 (see pp. 79-84, 132). 



The return pathway for motor nervous impulses from the 

 cerebral cortex is the cortico-spinal tract or pyramidal tract 

 (Fig. 64), whose fibers descend without interruption from the 

 precentral gyrus of the cerebral cortex (see p. 283) to the spinal 

 cord, where they form the lateral and ventral cortico-spinal 

 tracts (Fig. 59). The various reflex centers of the brain stem 

 also send motor fibers downward into the cord for the excitation 

 of movements of the trunk and limbs. The tecto-spinal tract 

 (Fig. 59) is such a path, leading from the optic and acoustic 

 centers of the midbrain, as is also the vestibulo-spinal tract, 

 leading from the vestibular nuclei of the medulla oblongata 

 (p. 176, Fig. 83, neuron 16). 



Summary. The spinal nerves are segmentally arranged and 

 are named after the vertebrae adjacent to which they emerge 

 from the spinal canal of the vertebral column. Each nerve 

 arises by a series of dorsal rootlets afferent in function and a 

 series of ventral rootlets efferent in function. Most of the gray 

 matter of the spinal cord is massed in two longitudinal columns 

 on each side, for somatic sensory and somatic motor functions 

 respectively. These are separated by an intermediate region 

 containing the visceral sensory and motor centers and various 

 correlation neurons. The white matter of the cord is superficial 

 to the gray and contains myelinated fibers for various kinds of 

 correlation, besides root-fibers of the spinal nerves. The white 

 matter is divided topographically into funiculi and fasciculi and 



