STEUOTUEE OF THE SPIJSTAL COED. 71 



mechanisms for producing the movements which in life were repeated num- 

 berless times. In the experiments cited these mechanisms, once stimulated 

 to activity, perform simple or successive combinations of movements in an 

 exactly normal way. 



The stimuli which reach the spinal cord from without — i.e., those which 

 reach it through the sensory nerves — are alone sufficient to produce much 

 that was formerly supposed to be possible only through higher psychical 

 processes. 



The activity of the spinal cord can be influenced, regulated, inhibited, 

 or stimulated by other parts of the central system. Let us consider, for a 

 moment, those tracts which are adapted to exert the enumerated influences 

 upon the activity of the cord. 



The author's own investigations justify the statement that from the 

 selachians and bony fishes to the mammals a few tracts are constant. In the 

 first place, the spinal cord is always in communication with the cerebellum. 



The Tractus cerebello-spinalis in mammals, probably also in birds and 

 reptiles, lies in the periphery of the lateral column. In fishes the author 

 has traced it posteriorly, but did not clearly make out its location in the 

 columns of the spinal cord. There is, however, ground for the opinion that 

 here, also, they lie in the lateral column and may be recognized again in the 

 thick fibers which are to be seen in the lateral columns of the spinal cord of 

 Gymnotus (Fig. 31). 



Then there is always found a tract which arises from the depths of the 

 Thalamencephalon and passes to the anterior columns. It has long since 

 been known in the mammals, where it receives the name Fasciculus longi- 

 tudinalis posterior (Fig. 44). 



Finally it may be accepted as highly probable that a large tract which 

 arises jn the roof of the Mesencephalon, in the tectum opticum, passes into 

 the anterior lateral column. In these fibers, which in fishes and birds are 

 especially numerous at their place of origin, one has probably to deal with a 

 central sensory tract. At its origin it is called deep midbrain-marrow; in 

 its later course it is called the fillet, or lemniscus. In the spinal cord one may, 

 with all certainty, recognize that fibers arise from this fillet where sensory 

 nerves end. They arise from those cells around which the nerve-roots from 

 the posterior ganglia ramify. It was possible, also, to demonstrate, regard- 

 ing the spinal cord, that for those cells in the gray substance around which 

 the dorsal roots ramify axis-cylinders arise which, after decussation in the 

 ventral commissure, pass toward the brain in the anterior and lateral 

 columns. These fibers arising from commissural cells are not yet with cer- 

 tainty to be differentiated. It is, however, probable, on clinical and experi- 

 mental grounds, that there is a crossed sensory tract in the lateral columns, 

 though the conclusive anatomical demonstration of it is not yet accom- 



