THE BRAIN 



695 



Fibre of 

 ^'&stibut,ar 



part most of the fibres cross the middle line, to become the crossed 

 pyramidal tracts of the cord, thus forming the motor decussation. 



Tracts which may perhaps be 



, i A ' v AI. Motor 



grouped as conducting in both nuclei 



directions are ,*r\ 



1. The olivo-cerebellar fibres, 

 which connect the inferior olive to 

 the cerebellum. 



2. The posterior dorsal longi- 

 tudinal bundle fibres, in which run 

 fibres in both directions between 

 the medulla and the anterior basis 

 bundle of the cord, and the pons and 

 mid-brain. This tract lies dorsally 

 to the mesial fillet, just below the 

 central canal and fourth ventricle. 

 The chief fibres of this tract come 

 (1) from the nuclei of the third 

 and sixth nerves, being concerned 

 in the regulation of eye move- 

 ments, and by way of the seventh 

 nerve in the movements of the 

 accessory apparatus, such as the 

 eyelids and eyebrows ; (2) from 

 Deiters' nucleus to the cord in con- 

 nection with the equilibration of 

 the body ; (3) from the twelfth 

 nucleus by way of the seventh 

 nerve to the orbicularis muscle of 

 the mouth. 



In close association ventrally 

 with the posterior longitudinal 

 bundle (sometimes classed as part 

 of it) is the anterior longitudinal 

 bundle, or tecto-spinal tract. In 

 it run fibres from the roof of the 

 mid-brain to the cord. 



The Functions of the Medulla 

 Oblongata. The medulla, like the 

 .spinal cord, acts as a conductor 

 and as a reflex centre. The centres 

 are those associated with the 

 functions of the nerves arising 

 from it, of which the vagus nerve 

 is the chief. Here, therefore, are 

 situated the centres concerned in 



the regulation of the heart-beat (the cardio-motor centre), the regu- 

 lation of the peripheral resistance (the vaso-motor centre), 



1\ 



.17 



J4 



- 



Ventral 

 horn 



of 

 cord 



FIG. 424. DIAGRAM OF A FIBRE OF THE 

 POSTERIOR LONGITUDINAL BUNDLE 

 ARISING PROM A CELL OF DEITBES' 

 NUCLEUS. (E. A. Schafer, from 

 " Qitain's Anatomy.") 



the 



