36 BRAIN AND BODY OF FISH 



area of the brain. The series begins at the bottom of the Plate 4, 

 and Fig. i is a section of the hinder end of the medulla across the 

 vagal lobes, where they are most prominent. These are separated 

 by a deep cleft, the rhomboid fossa, which is the opening out of the 

 fourth ventricle. At the base of the fossa on either side there is a 

 group of large motor cells, known as the nucleus ambiguus from 

 which pass the efferent fibres of the motor root of the Xth or vagal 

 nerve. External to this motor nucleus on either side is a large 

 bundle of longitudinal fibres cut transversely, which are a prominent 

 featiu"e of the medulla and can be traced in four of the sections of 

 the plate. This bundle is known as the great longitudinal secondary 

 gustatory tract, and in it can be distinguished three divisions, an 

 upper (that is most dorsal), known as the spinal root of the fifth 

 nerve, a middle known as the descending gustatory tract which 

 receives fibres from the facial lobe, and the lower the ascending 

 secondary gustatory tract. In the middle line just below the rhom- 

 boid fossa are a number of bundles of nerve fibres known as the 

 longitudinal median bundles (or the fasciculus medialis). 



Fig. ii is a section somewhat anterior to the preceding and the 

 vagal lobes no longer appear. In the site of the rhomboid fossa is 

 now seen the facial lobe, and below it is the central canal or ventricle. 

 On either side of the central lobe we see the commencement of the 

 fifth lobes. These represent the skin areas, so are usually called the 

 somatic -sensory lobes. Descending fibres pass on either side into 

 the gustatory tract from the facial lobe, and crossing these descend- 

 ing fibres pass from the fifth lobes to cross each other tlu"ough the 

 median longitudinal bundle. The facial nerves are also to be seen 

 at the base of the facial lobe into which their fibres gradually pass. 



Fig. iii is very similar to the section which has been just des- 

 cribed, but the fifth lobes are much more prominent and almost 

 entirely embrace the facial lobe, which is much diminished in size. 



Fig. iv. — The facial lobes are replaced by the two trunks of the 

 facial nerves which are passing backwards to enter the lobe as was 

 seen in Fig. iii. 



Fig. V. — Here the cerebellum first appears and is joined to the 

 medulla laterally by the acoustic tubercles. The facial nerves are 

 now cut longitudinally and appear to pass transversely towards the 

 ventricle ; the fifth also are seen on either side. 



If we study a similar series of sections of the madulla oblongata 

 of Engraulcypris, a very different picture will be observed. We 

 have seen that the naked eye appearance of the medulla shows the 

 absense of a facial lobe as seen in the roach, and the vagal lobes are 

 not easily identified. c 



