STRUCTURE OF THE MEDULLA OBLONGATA. 505 



and establish a communication between the motor tract at the base of 

 the Encephalon (which is chiefly derived from the Corpora Striata) and 

 the anterior and antero-lateral columns of the Spinal Cord. They 

 have also a connexion with the Cerebellum. A large part of its fibres 

 decussate, those that proceed from the right hemisphere passing into 

 the left side of the cord, and those from the left hemisphere into the 

 right side of the cord ; an arrangment which fully explains the fact, 

 that in Hemiplegia, the paralytic affection of the body is on the side 

 opposite to that of the lesion of the brain. A small proportion of the 

 fibres of the anterior pyramids does not decussate ; and this passes 

 down, with fibres from the olivary columns, into the anterior columns 

 of the cord ; whilst the decussating fibres dip more deeply away from 

 the anterior surface of the cord, and connect themselves rather with its 

 lateral or middle columns. 



891. The fibrous portion of the Olivary body is connected above with 

 the Motor tract, with the Corpora Quadrigemina, and with the Cere- 

 bellum, and below with the anterior columns of the Spinal Cord. The 

 vesicular nucleus of the Olivary body, on the other hand, which is 

 known as the corpus dentatum, seems to be especially connected with 

 the origins of the nerves concerned in the regulation of the movements 

 of the tongue ; thus we find that anteriorly a portion of the roots of 

 the hypoglossal, which is the motor nerve of the tongue, issue from 

 it ; whilst posteriorly, a portion of the roots of the glosso-pharyngeal, 

 which is one of the sensory nerves of the tongue, seem to terminate 

 in it. 



892. The fibres of the Restiform columns are continuous above with 

 those of the hemispheres of the Cerebellum ; and below they pass, 

 without decussation, chiefly into the posterior columns of the Spinal 

 Cord, a band of arciform fibres, however, crossing over to the anterior 

 and lateral columns on each side. The ganglia imbedded in these 

 columns, however, seem to possess a completely independent function ; 

 being the centres of the Pneumogastric nerves, which are the chief ex- 

 citors of the Respiratory movements, as well as of a portion of the 

 Glosso-pharyngeal nerves. 



893. The Posterior Pyramids are two small strands of fibrous struc- 

 ture, lying between the two restiform bodies, and occupying the portion 

 of the Medulla Oblongata on either side of the posterior median furrow. 

 They may be traced upwards into the Thalami Optici, and downwards 

 into the posterior columns and the posterior part of the lateral. They 

 undergo a decussation in their upward course ; but it is not certain 

 whether this decussation involves all their fibres. The gray nuclei of 

 the Posterior Pyramids, which are situated immediately beneath the 

 fourth ventricle, are the ganglionic centres of the Auditory nerves, or 

 the proper Auditory ganglia. 



894. When we consider these various lines of communication simply 

 in their Physiological relations, as establishing connexions between the 

 Encephalon above and the Spinal Cord below, it will be convenient 

 first to notice and put aside the Cerebellar. Of these there are two 

 sets ; the principal forming the Restiform bodies, which connect the 

 Cerebellum with the posterior columns of the Spinal cord ; whilst there 



