86 



ANATOMY OF THE CENTEAL NBHVOUS SYSTEM. 



visible only as an elongated prominence, and in birds and mammals it forms 

 only a slight prominence on the floor of the fourth ventricle (compare also 

 Pig. 52). 



From the ventral side fibers enter the vagus nucleus -which cross over 

 from the opposite side. The decussation is so striking in fishes that one 



Fig. 44. — Sagittal section through brain, medulla, and upper cord of a young 

 cyprinoid of four-centimeters' length, showing the course of the Fasciculus longi- 

 tudinalis posterior. 



may readily see its white fibers with the imaided eye. We have to deal here 

 with a fascicle to the fillet. The nu.clei of all cranial nerves possess such 

 (Fig. 45, Tr. var/o-tect.). Furthermore, the sensory nucleus of the Yagus 

 receives an accretion (especially large in fishes) from the cerebellum where 

 decussation takes place. 



In the higher vertebrates, also, there exists a vago-cerebellar tract, but 



Fig. 45. — Two sections from the medulla of a young gold-fish — Cypriims auratus- 

 four centimeters long. Sec. A is the more posterior of the two. 



in mammals at least it passes out with the root-fibers direct. In fishes and 

 amphibians a part of the fibers of the Vagus innervate the skin just like 

 sensory spinal nerves; but in the higher vertebrates the sensory part of the 

 vagits contains only sensory nerves from the viscera. The Vagus contains 



