206 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



lemniscus), which explains the crossed sensory control, as the 

 pyramidal decussation explains the crossed motor control of 

 the cerebrum in relation to the body. After this decussation 

 the sensory fibers form a longitudinal bundle on each side known 

 as the median fillet or lemniscus, which in the pons lies just 

 dorsal to the pyramidal system of fibers. 



The lemniscus fibers may be traced forward (see Fig. 91) as 

 far as the superior colliculus of the corpora quadrigemina and 

 the thalamus, the important termination being in the thalamus 

 (ventral or lateral nucleus). Those neurons that end in the 

 thalamus are continued forward by a third set of neurons, which 

 end in the parietal lobe of the cerebrum (see Fig. 82, C). On its 

 way through the medulla and pons the lemniscus is believed to 

 receive accessions of sensory fibers from the sensory nuclei of 

 the cranial nerves of the opposite side. The course of the lem- 

 niscus has been traced by various means, but especially by the 

 method of myelinization during embryonic life and by degenera- 

 tion consequent upon long-standing disuse. As was stated in 

 the section upon Nerve Degeneration, injury to an axon is 

 followed quickly by degeneration of the peripheral end, and 

 much more slowly by a degeneration of the central end and the 

 nerve cell itself, when the path is not again established. Certain 

 long-standing cystic lesions (porencephaly) in the parietal cor- 

 tex have resulted in an atrophic degeneration of the lemniscus 

 fibers, thus adding materially to the evidence that this sensory 

 tract ends eventually in the region indicated.* Further evidence 

 of the same character is found in the observations made by 

 Campbellf upon cases of tabes dorsalis. The lesion in such cases 

 is in the posterior funiculi of the spinal cord, but eventually 

 the whole upward path is affected and degenerative changes are 

 found in the cells of the posterior central convolution. 



From the connections of the lemniscus with the tracts of the 

 posterior funiculi of the cord it is evident that it forms one 

 pathway at least for the fibers of muscle sense. Whether or not 

 the fibers of pressure, pain, and temperature take the same route 

 is not known, but it seems probable, at least, from the known 

 connections of the lemniscus with the sensory nuclei of the 

 cranial nerves and with the sensory tracts of the lateral as well 

 as the posterior funiculi of the cord. The lemniscus ends 

 chiefly in the thalamus, before passing on to the cortex, and 

 here, as in other similar cases, we have the possibility that the 

 lower centers, in addition to the reflex connections which they 

 make, may mediate also some form of conscious reaction. 

 While the general tendency has been to confine the conscious 



*H6sel, "Archiv f. Psychiatric, " 24, 452, 1892. 



t Campbell, ' ' Histological Studies on Localisation of Cerebral Functions, " 

 Cambridge, 1905. 



