v. io RESPIRATION 157 



afferents and they run backwards through the brain tissue to the 

 pre-optic nucleus of the hypothalamus. A possible clue to its origin is 

 that this is the region of the brain where the morphologically ventral 

 region of the neuraxis ends (p. 147). The nervus terminalis may repre- 

 sent the ventral olfactory nerve, the much larger main nerve being 

 morphologically dorsal. 



A further puzzle of some importance which may be mentioned here 

 is the course of the proprioceptor fibres for those muscles that are 

 supplied purely by ventral roots. The eye-muscles contain proprio- 

 ceptor organs and Sherrington and others have shown that the affer- 

 ent fibres connected with these run to the brain through the third, 

 fourth, and sixth nerves, that is to say, through ventral roots. Simi- 

 larly, it has been shown that there are afferent fibres in the hypo- 

 glossal nerves in mammals. Conversely it is now known that there are 

 efferent fibres running from the brain to many receptor organs. For 

 example, such fibres run in the auditory nerve. To pursue these ques 

 tions farther would lead us into discussion of the factors that control 

 the making of connexions within the nervous system Here we are 

 concerned only with analysis of the plan that produces the main out- 

 lines of the structures in the head, a plan which, with all its modifica- 

 tions, is essentially segmental. 



10. Respiration 



The function of the branchial arches is not merely to support the 

 gills but to allow the movements of the pharynx wall by which the 

 respiratory current of water is produced. It is for this reason that 

 the jointed system of rods is present. The respiratory movements con- 

 sist in a lowering of the floor of the mouth by means of the hypo- 

 glossal muscles, with at the same time an expansion of the walls of 

 the pharynx. This causes an inrush of water through the mouth, 

 which is then closed and the floor raised, forcing the water out through 

 the gill-slits. The whole movement is worked by the 'visceral' (lateral 

 plate) muscles of the pharynx wall, innervated by the trigeminal, 

 facial, glossopharyngeal, and vagus nerves, in co-operation with the 

 myotomal hypoglossal muscles, innervated by the hypoglossal nerve. 



The gill filaments bear lamellae that meet at the tips, leaving minute 

 channels for the water. The blood flows through the lamellae in the 

 opposite direction to the water so that just before leaving the gills 

 the blood meets the highest concentration of oxygen and lowest of 

 carbonic acid. 



