FISHES. 331 



dibulum (f), and in most fishes without uniting or sticking to each 

 other, their connexion being by mere cellular tissue. 



In the perch, or the cod for example, it is easy to uncross them 

 and see that the nerve of the right eye comes from the left side of 

 the medulla, and vice versa. But in the Ray their union is so inti- 

 mate, that their crossing is as problematical as it is in uiammalia. 



In a certain number of osseous fishes, their structure is very 

 remarkable, inasmuch, as their medullary substance is no more than a 

 large thin ribband folded longitudinally to fill the tube with which it 

 is furnished by the dura mater ; but there arc other fishes in which 

 it is composed as usual, of a bundle of nervous threads. 



We have already pointed out the origin of the third {p), and 

 fourth (9), pairs. 



The fifth, or trigeminal nerve (r), arises from the sides of the 

 fourth ventricle, below and close to the anterior part of the lobes ((/), 

 placed behind the cerebellum, or the crura of the cerebellum itself. 

 The roots of it can be traced more deeply and in different directions. 



The origins of the eight pair, or par vagum (/), which arc almost 

 as considerable as those of the fifth pair, are behind the latter ; they 

 arise most frequently in several filaments which come out on a single 

 longitudinal line, on the sides of the medvdla, under the lobes behind 

 the cerebellum ; they form a ganglion (^), after which they arc sub- 

 divided. 



Between the fifth and eight pairs, we recognise the acoustic nerves 

 (.y, ,?), and there is usually, in front of the eighth pair, a particular 

 nerve {v,v), which answers to the glosso-pharyngeal. The ninth 

 pair is not found in fishes. _ , . 



The spinal nerves, reckoning from the tenth pair, arise as in the 

 superior classes, from the spinal marrow by two sets of roots ; but 

 they do not always arise near the foramina in the vertebral column, 

 through Avhich they pass. There are even species, such as tlic 

 Moon-fish, in which the spinal marrow is so shortened as to seem_ to 

 be no more than a small conical prominence of the brain, from which 

 the different pairs of nerves go out as from a horse's tail.* 



In others, such as the lump-fish, it is prolonged and swelled oppo- 

 site each pair. In general, it does not terminate till near the end of 

 the spine. The nerves of the superior roots have scarcely any sen- 

 sible SAvelling as a ganglion in the chondropteiygians ; it has been 

 even denied that they have any swelling whatever in osseous fishes. 

 It is, however, certain that they form sufficiently well marked ganglia 

 in the Bar, the Perch, &c. „ , , • x 



The first pairs of spinal nerves are collected more or less into a 

 plexus, to be sent ofl'to the pectoral fin. In the Ray, in which the 

 finis so enormous.it receives nerves from many other parts of the spine. 



The great sympathetic nerve (x x, fig. iv.) derives its roots, as 

 usual from the different spinal nerves, and forms divers plexuses 

 and ganglia in its distribution to the viscera. In general, its tenuity 



* It is utterly wrong, as Arsaky and others on his authority have said, that there 

 is nothing like tlxis in the lophius ; its spinal marrow measures almost the whole 

 length of the spine, but it is enveloped and concealed by the nerves, which arise 

 much higher than they go out. 



