. 



NERVOUS SYSTEM OF VERTEBRATA. 491 



immediate relation with the Encephalon, is termed the Medulla Oblon- 

 gata. It is in this, that the centres of the respiratory and stomato- 

 gastric nerves are found ; the situation of these important ganglia within 

 the cranium, being obviously destined to protect them from those inju- 

 ries, to which the Spinal Cord itself is liable. 



868. Thus, then, we are led to recognise in the Nervous system of 

 Vertebrata the following fundamental parts. 1. A system of ganglia 

 subservient to the reflex actions of the organs of locomotion, and cor- 

 responding with the chain of pedal or locomotive ganglia that makes 

 up the chief part of the ventral cord of the Articulata ; in this system, 

 the gray or vesicular matter forms one continuous tract, which occupies 

 the interior of the Spinal Cord. 2. A ganglionic centre for the move- 

 ments of respiration, and another for those of mastication and degluti- 

 tion ; these, with part of the preceding, make up the proper substance 

 of the Medulla Oblongata. 3. A series of ganglia, in immediate con- 

 nexion with the organs of Special Sense ; these are situated within the 

 cranium, at the anterior extremity of the Medulla Oblongata ; and, in 

 the lowest Vertebrata, they constitute by far the largest portion of the 

 entire Encephalon. 4. The Cerebellum, which is a sort of off-shoot 

 from the upper extremity of the Medulla Oblongata, lying behind the 

 preceding. 5. The Cerebral Hemispheres, a pair of ganglionic masses, 

 which lie upon the ganglia of special sense, capping them over more 

 or less completely, according to their relative development. These last 

 two organs exist in the lowest Vertebrata, as in Invertebrated animals 

 generally, in quite a rudimentary state ; but their development, rela- 

 tively to other parts of the Encephalon, and to the entire bulk of the 

 animal, increases as we ascend the scale ; so that in Man and the higher 

 Mammalia they constitute by far the largest portion of the Nervous 

 centres, and are essential to the greater part of the operations of the 

 Nervous system. The development of the Cerebral Hemispheres holds 

 a close relation with the increase of the Intelligence, and with the pre- 

 dominance of the Will over the involuntary impulses. The increased 

 size of the Cerebellum, on the other hand, seems connected with the 

 necessity which exists, for the adjustment and combination of the loco- 

 motive powers, when the variety in the movements performed by the 

 animal is great, and a more perfect harmony is required among them. 

 A sketch of the mode in which these different parts are combined 

 and arranged in the several classes of Vertebrata, and of their relative 

 development in each, will aid us in the subsequent more detailed exami- 

 nation of their functions. 



869. In the class of Fishes, taken as a whole, the Encephalon bears 

 a much smaller proportion to the Spinal Cord, than in the higher Ver- 

 tebrata. In the curious Amphioxus, or Lancelot, there is no discover- 

 able nervous mass anterior to the Medulla Oblongata ; and we have here, 

 therefore, an animal regularly formed upon the plan which occasionally 

 presents itself as a monstrosity in Man, namely, having the Spinal 

 Cord and Medulla Oblongata for the whole of the nervous centres, and 

 being anencephalous, or destitute of any proper encephalon. In some 

 of the lowest Vermiform (worm-like) Fishes, such as the Lamprey, the 

 cephalic masses are very little more developed in proportion to the 



