iv. 9 BRAIN OF LAMPREYS 101 



directly into the ventral roots; probably they do not do so but the 

 dendrites of the motor-cells branch around them and thus receive 

 stimulation (Fig. 63). In the earliest larva co-ordination is by a pair of 

 giant Mauthner cells, with dendrites among the entering fibres of the 

 eighth nerve and an axon descending on the opposite side. Such cells 

 are present in the earliest stages of nearly all fishes and amphibians. 



Other nerve-cells in the more dorsal parts of the cord have no long 

 axons and apparently serve to connect the neuropil of the various 

 regions. The afferent fibres reaching the cord in the dorsal roots give 

 off branches that ascend for a short distance and descend for long 

 distances. The pathways to the brain thus pass through multiple 

 relays. 



The brain itself (Fig. 64) is built on the typical vertebrate plan, as 

 an enlargement of the front end of the spinal cord, with thickenings 

 and evaginations corresponding to the various organs of special sense. 

 Although we know little of its internal functional organization in 

 lampreys, it is probably not far wrong to regard it as chiefly consisting 

 of a series of hypertrophied special sensory centres; thus the forebrain 

 is connected with smell, midbrain with sight, hind-brain with 

 acoustico-lateral and taste-bud systems. The forebrain and olfactory 

 sense are moderately well developed in adult lampreys, as is the visual 

 sense, with its chief centre in the midbrain. The auditory and acoustico- 

 lateral systems are not very well marked, and the cerebellum is small. 

 Taste is also much less developed than in the higher fishes (p. 220). 



Parts of the brain 



Forebrain (prosencephalon) Cerebral hemispheres (telen- 



cephalon) 

 Between-brain (diencephalon) 

 Midbrain (mesencephalon) Optic lobes 



Hind-brain (rhombencephalon) Cerebellum (metencephalon) 



Medulla oblongata (myelen- 

 cephalon) 



The upper surface of the brain is covered by an extensive vascular 

 pad, the choroid plexus or tela choroidea (Fig. 64). This extends into 

 the ventricles of the brain at three points — into the third ventricle of 

 the diencephalon, into the iter (duct) leading through the midbrain 

 from third to fourth ventricles, and into the fourth ventricle itself. 

 The roof of the brain is thus non-nervous in these regions. In later 

 vertebrates the choroid extends only into the third and fourth 



