NERVOUS SYSTEM 



853 



example with which to work. It has no cerebral cortex developed into 

 immense hemispheres, as in man, and which make it so difficult to study 

 the underlying parts and note their relationship (Figs. 477, 478). 



There is a regular system of small sensory canals widely distributed 

 in fishes containing sense-organs slightly similar to those in the semi- 

 circular canals of the internal ear. Their functions are supposed to be 

 somewhat between that of organs of touch in the skin and those of 

 equilibrium of the internal ear. The water vibrations of slow frequency 

 probably make it possible for the animal thus to orient itself. Their 

 innervation comes/from the VII, IX, and X pairs of cranial nerves. The 

 sensory canals just mentioned are called lateral line organs (Fig. 479) 

 and are absent in higher vertebrates. 



If Figure 480 be studied carefully it will be seen that there is a 

 quite definite area or center for each group of impressions. 



The acoustisco-lateral area is the terminal center of the lateral line 

 nerves as well as the acoustic nerve (VHI pair). 



The general cutaneous area receives impressions from the remain- 

 ing general exterior of the body. 



The nerves from the viscera (that is, from the gills, stomach, etc.) 

 enter a visceral area. 



The eye is connected with the optic lobe. 



n. am- 



pal. b.' / ^'' i 



ing 



Fig. 477. 



A semi-diagrammatic drawing of a longitudinal section through a dogfish, 

 passing slightly to the right of the middle line, a.ch.p., anterior choroid plexus ; 

 a./., anterior fontanelle ; au., auricle ; au.v., auriculo-ventricular opening and 

 valve ; b.b., basibranchial cartilage ; b.h., basihyal cartilage ; c., centrum ; e.a., 

 conus arteriosus ; cb., cerebellum ; cer., cerebrum ; cor., coracoid region of the 

 pectoral girdle ; gr., grooves in which the teeth are formed ; i.p., intercalary 

 plate ; inf., infundibulum ; lat.v., lateral ventricle ; M.c., Meckel's cartilage ; n.a., 

 neural arch ; n.am., ampullary sense organs ; n.sp., neural spine ; nch., notochord ; 

 oes., oesophagus ; op.l., optic lobe ; p.ch.p., posterior choroid plexus ; p.st., pineal 

 stalk ; pal.b., palatine bar ; pm., pericardium ; pp.c., pericardio-peritoneal canal ; 

 s.au., sinu-auricular opening ; s.v., sinus venosus ; sp.c., spinal cord ; st., semilunar 

 valves ; tng., tongue ; v., ventricle ; v.ao., ventral aorta ; 3, third ventricle ; 4, fourth 

 ventricle. (From Borradaile.) 



