THE MEDULLA OBLONGATA. 



1071 



Nucleus cuneatus 



Nucleus 



gracilis 



Fibres from 



Coil's tract Fibres from Burdach's tract 

 Post, superficial arcuate 



of the pons and cerebral crura. A feeble expression of a somewhat similar structure 

 is seen in the reticular formation within the lateral column of the spinal cord. 



The Arcuate Fibres. These originate as the axones of the cells of the gracile 

 and cuneate nuclei and include three sets. The first, the deep arcuate fibres, turn 

 sharply brainward after 



crossing the raphe and FIG. 923. 



constitute the chief con- 

 stituents of the mesial 

 fillet. The second set, the 

 anterior superficial 

 arcuate fibres, also 

 cross the mid-line, but 

 these, instead of turning 

 upward, pass forward, 

 enter through the pyra- 

 mid or along its median 

 aspect, and, gaining the 

 surface, sweep over the 

 pyramid and olivary emi- 

 nenceand thenceproceed 

 backward to the restiform 

 body and on to the cere- 

 bellum. An oval collection of small fusiform nerve-cells, the arcuate nucleus 

 (nucleus arcuatus) lies in the path of these fibres, at first on the ventral surface of the 

 pyramid and then along the median fissure. Whilst some additional arcuate fibres 

 arise from the cells of the nucleus, the majority sweep by without interruption. 

 The third set, the posterior superficial arcuate fibres, proceed from the cells of 

 the gracile and cuneate nuclei of the same side and pass beneath the ventricular 

 floor to the adjacent restiform body and thence to the cerebellum. 



Deep arcuate 



Anterior superficial arcuate 



Arcuate nucleus 



Diagram illustrating source and path of arcuate fibres; RB, restiform body; 

 P, pyramidal tract ; O, inferior olivary nucleus. 



FIG. 924. 



Nucleus gracilis 



Nucleus cuneatus internus 



Nuc. cuneatus 

 'externus 



Funiculus cuneatus - 



Fasciculus solitarius 



Nucleus lateral! 



Nucleus ambiguu 



Decussation of 

 fillet fibres 



Median fillet 



Pyramidal tract 



Anterior superficial arcuate fibres 



5\ Deep arcuate fibres 

 33- and f ormatio 

 fflj reticularis 



Dorsal access, 

 olivary nucleus 



Inferior olivary 

 nucleus 



Mesial access, 

 olivary nucleus 



Root-fibres of 

 hypoglossal 



Arcuate nucleus 



Transverse section of medulla at level D, Fig. 919, showing posterior nuclei, inferior olivary nuclei, formatio 

 reticularis and dorsal displacement of central canal. X 5%. Preparation by Professor Spiller. 



The Olivary Nuclei. These include, in each half of the medulla, three masses 

 of gray matter the inferior olivary nucleus and the two accessory olivary nuclei. 

 Beneath the prominent olivary eminence lies a corrugated sack-like lamina of gray 



