CRANIAL NERVES. SYMPATHEMIC. 75 



For an account of a typical arrangement of the cranial nerves 

 the reader is referred to the section on Elasniobranchii. Their 

 arrangement in fishes differs from that in higher types, mainly 

 on account of the presence of the lateral line sense organs. The 

 nerves to these appear to arise from a special part of 

 the brain, the tuherculum acusticum, from which the auditory 

 nerves also arise. They are associated m their course to the 

 periphery witli the seventh and tenth nerves, and constitute 

 the acustico-lateralis system. The fibres of this system which 

 run with the fifth and ninth are derived from these two nerves. 

 The nerves which pass from the facial roots to the fifth nerve 

 cause an intermingling of the roots of these two nerves, which 

 is not easy to unravel, and which is characteristic of fishes. 



A sympathetic nervous system appears to be present. In 

 Marsipobranchii, in which all the nerves are without a medullary 

 sheath, it cannot be fully traced, but the spinal nerves give off 

 branches which pass to the viscera, where small ganglia are 

 found. In other Pisces there is a series of sympathetic ganglia 

 which develop as outgrowths of the spinal nerves, becoming 

 detached from the rudiments of the spinal ganglia at an early 

 stage. These ganglia are usually connected by longitudinal 

 commissures, but though regularly developed, their arrange- 

 ment is not easy to trace in the adult. In Elasmohranchii * 

 the system tends to take a plexiform structure, and lies in the 

 neighbourhood of the cardinal veins. There is an especially 

 large ganglion at about the level of the ductus cuvieri ; this is 

 supplied by a number of spinal nerves, and gives off several 

 branches, which are distributed to the viscera with the coeliac 

 artery. The system appears not to extend into the head. In 

 Teleostei there is a definite chain of small ganglia on each side 

 of the vertebral column. In these forms it is continued into 

 the head, wliere it is connected with the trigeminal nerve and 

 ciliary ganglion, and into the tail, where it runs in the caudal 

 canal. 



The analysis of the nerves, which is the outcome of the recent work f 

 of morphologists and physiologists, is beyond the scope of this work, 

 but the following points may be noted here : 



* R. Chevrel, " Sur Fanatomiedusystemenerveuxgrandesympathetique 

 des Elasmobranches et des poissons osseux." Arch. Zool. Exf. (2) 5, 

 supplement. 



t W. H. Gaskell, "The structure and f miction of the nerves which inner- 



