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492 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



Spinal Cord, than are the cephalic ganglia of Insects in reference to 

 their chain of ventral ganglia. But as the organs of special sense ac- 

 quire a more complete evolution, we find the ganglia connected with 

 them presenting a greatly-increased size. On opening the cranial 

 cavity of a Fish, we usually observe four nervous masses (three of them 

 in pairs) lying, one in front of the other, nearly in the same line with 

 the Spinal cord. The first or most anterior of these are the Olfactory 

 ganglia (Plate II., Figs. , 6, 7, a\ or the ganglia of the nerves of 

 smell; the nature of which is known, from their being situated at the 

 origin of the Olfactory nerves. In the shark and some other Fishes, 

 these are separated from the rest by peduncles or foot-stalks ; a fact 

 of much interest, as explaining the arrangement which we find in Man. 

 What is commonly termed the trunk of his Olfactive nerve is really the 

 commissure connecting the Olfactive ganglion (known as the bulbous 

 enlargement that lies upon the cribriform plate of the Ethmoid bone) 

 with the other portions of his Encephalon : the proper fibres of the 

 nerve being those which come off from this ganglion, in the numerous 

 branches that proceed from it into the nasal cavity. Behind the Olfac- 

 tive ganglia is a pair of masses, 6, , of which the relative size varies 

 greatly in different Fishes. Thus in the Perch, whose Encephalon is 

 here figured, their size is intermediate between that of the first and 

 third pairs ; being as much inferior to that of the third, as it is superior 

 to that of the first. On the other hand, in the Shark and several other 

 Fishes, they are considerably larger than the succeeding pair. These 

 second ganglia are commonly considered as the rudiments of the Cere- 

 bral Hemispheres ; but there seems reason for regarding them as being 

 chiefly the representatives of the Corpora Striata ; the existence of a 

 cerebrum being only indicated by a thin layer of vesicular matter, 

 which overlies the ventricle that is found in these bodies in the brains 

 of ^ Cartilaginous Fishes alone. Behind them, and forming the third 

 pair of ganglionic masses, c, <?, are two large bodies, from which the 

 optic nerves arise ; these evidently represent the Optic ganglia, which 

 constitute the principal mass of the cephalic ganglia in Insects and the 

 higher Mollusca, and with which the Corpora Quadrigemina of higher 

 Vertebrata partly correspond; but they probably represent also the 

 Thalami Optici of the brain of Man and the higher animals. At the 

 back of these, overlying the top of the spinal cord, is a single mass, d, 

 the Cerebellum. This, also, varies greatly in its relative dimensions, 

 being much more highly developed in the active and rapacious Sharks, 

 than it is in Fishes of inferior muscular energy and variety of move- 

 ment. -The Spinal Cord 0, is divided at the top by a fissure, which is 

 most wide and deep beneath the cerebellum, where there is a complete 

 separation between its two halves. This opening corresponds to that, 

 through which the oesophagus passes in the Invertebrata ; but as the 

 entire nervous mass of Vertebrated animals lies above the alimentary 

 canal (or nearer the dorsal surface), it does not serve the same purpose 

 in them ; and in the higher classes, the fissure is almost entirely closed 

 by the union of the two halves on the median plane, the fourth ven- 

 tricle, however, being a remnant of it. This cavity is partly seen in 

 Fig. 7, which is a vertical section of the brain whose upper and under 



