SENSORY IMPULSES AND THE MEDULLA OBLONGATA. 



655 



abolishes sensibility on the opposite side below the lesion. There is hyperesthesia 

 of the parts below the seat of the section on the side of the injury ( 363). From 

 experiments on mammals, Brown-Sequard concludes that the decussating sensory 

 nerve-fibres pass to the opposite side within the cord 

 at different levels, the lowest being the fibres for 

 touch, then those for tickling and pain, and, highest 

 of all, those which administer to sensations of tem- 

 perature. 



All the fibres, therefore, which connect the spinal 

 cord with the grey matter of the brain, undergo a 

 complete decussation in their course. Hence, in man 

 a destructive affection of one hemisphere usually causes 

 complete motor paralysis and loss of sensibility on the 

 opposite side of the body. The fibres proceeding from 

 the nuclei of origin of the cranial nerves also cross 

 within the cranium. 



Not unfrequently the motor paralysis and anaesthesia occur 

 on the same side of the head, in which case the lesion (due to 

 pressure or inflammation) involves the cranial nerves lying at 

 the base of the brain. 



The positions of decussation are (1) in the spinal cord, (2) 

 in the medulla oblongata, and lastly (3) in the pons. The 

 decussation is complete in the peduncle. 



Alternate Paralysis. Gubler observed that unilateral in- 

 iury to the pons caused paralysis of the facial nerve on the _ 

 same side, but paralysis of tne .opposite half of the body. He Diagram of a spinal segment as 

 concluded that the nerves of the trunk decussate before they a spinal centre and conduct- 

 reach the pons, while the facial fibres decussate within the 

 pons. To these rare cases the name "alternate hemiplegia" 

 is given. [When haemorrhage takes place into the lower part 

 of the lateral half of the pons, there may be alternate para- 

 lysis, but when the upper part of the lateral half is injured, 

 the facial is paralysed on the same side as the body, 379.] 



The olfactory nerve is said not to decussate (?), while the 

 optic nerve undergoes a partial decussation at the chiasma 

 ( 344). Some observers assert that the fibres of the troch- 

 learis decussate at their origin. 



366. THE MEDULLA OBLONGATA. [Structure. In 



the medulla oblongata, the fibres from the cord are rearranged, 

 the grey matter is also much changed, while new grey matter 

 is added. Each half of the medulla oblongata consists of the 

 following parts, from before backwards : The anterior pyra- 

 mid, olivary body, restiform body, and posterior pyramid, 

 or funiculus gracilis (figs. 464, 465, 466). By the divergence 

 of the posterior pyramids and the restiform bodies, the floor 

 of the 4th ventricle is exposed. As the central canal of the 

 cord gradually comes nearer to the posterior surface of the 

 medulla, it opens into the 4th ventricle. At the lower end 

 of the medulla oblongata, on separating the anterior pyra- 

 mids, we may see the decussation of the pyramids, where the fibres cross over to the lateral 

 columns of the cord. The anterior pyramid receives the direct pyramidal tract of the anterior 

 column of the cord from its own side, and the crossed pyramidal tract from the lateral column 

 of the cord of the opposite side (fig. 464). The decussating fibres (crossed pyramidal tract) of 

 the lateral column pass across in bundles to form the decussation of the pyramids. Most of the 

 pyramidal fibres pass through the pons directly to the cerebrum, a few fibres pass to the cere- 

 bellum, while some join fibres proceeding from the olivary body to form the olivary fasciculus 

 or fillet.] 



[Thus, only a part of the anterior column of the cord direct pyramidal tract is continued 

 into the anterior pyramid, where it lies external to the fibres which pass to the lateral column 

 of the opposite side. The remainder of the anterior column the antero-external fibres are 

 continued upwards, but lie deeper under cover of the anterior-pyramid, where they serve to form 

 part of the formatio reticularis (p. 658).] 



Fig. 463. 



ing medium. B, right, B 

 left cerebral hemisphere ; MO, 

 lower end of medulla oblon- 

 gata; 1, motor tract from the 

 right hemisphere, the larger 

 part decussating at MO, and 

 passing down the lateral column 

 of the cord on the opposite side 

 to the muscles M and M' ; 2, 

 motor tract from the left hemi- 

 sphere ; S, S', sensitive areas 

 on the left side of the body ; 

 3', 3, the main sensory tract 

 from the left side of the body 

 it decussates shortly after 

 entering the cord ; S 2 , S 3 , sen- 

 sitive areas, and 4', 4, tracts 

 from the right side of the body. 

 The arrows indicate the direc- 

 tion of the impulses (Bramwell). 

 [Here all the sensory fibres are 

 shown as crossing in the cord.] 



