NERVOUS SYSTEM OP VERTEBRATA. 363 



is so much higher, and its importance so much greater, that 

 special care is taken to guard it from injury. The term brain 

 is commonly applied to the whole mass of nervous matter 

 contained within the cavity of the skull ; but this consists of 

 several distinct parts, which have obviously different charac- 

 ters. The principal mass in Man and the higher Vertebrata 

 is that which is termed the Cerebrum (fig. 195, a) ; this occu- 

 pies all the front and upper part of the cavity of the skull, and 

 is divided into two halves or hemispheres by a membranous 

 partition which passes from back to front along the middle 

 line. Beneath this, at the back part of the skull, is another 

 mass, b, much smaller, but still of considerable size, termed 

 the Cerebellum; and this also is divided into two hemi- 

 spheres. At the base or under side of the cerebrum, and 

 completely covered-in by it, are two pairs of ganglia (1 and g, 

 fig. 196), which belong to the nerves of smell and sight. We 

 shall presently find that these are, relatively speaking, much 

 larger in the lower Vertebrata than in the higher. 



450. The several masses of nervous matter contained in the 

 skull, are connected with each other and with the spinal cord 

 by bands of nerve-fibres and tracts of vesicular substance, 

 which serve to bring the brain into connexion with the nerve- 

 trunks issuing from the spinal cord. But the Spinal Cord 

 has also distinct properties of its own, analogous to those 

 which have been shown to exist in the chain of ganglia in 

 Insects. The upper part of it, which passes-up into the 

 cavity of the skull, is termed the Medulla Oblongata (/', fig. 

 197). This is connected with the nerves of respiration, masti- 

 cation, and deglutition; and may be regarded as combining 

 together the respiratory and the stomato-gastric systems of 

 Invertebrata. The remainder of the spinal cord, which de- 

 scends through the vertebral column, sends its nerves to the 

 limbs and trunk ; and may be regarded as analogous to the 

 chain of ganglia by which the corresponding parts are sup- 

 plied in Insects. 



451. The nerves which issue from the Spinal Cord, all 

 possess two sets of roots ; one from the anterior portion of 

 the cord, the other from its posterior portion (fig. 191). The 

 fibres which come-off by these two sets of roots, soon unite 

 into the trunk of the nerve, which thus possesses the proper- 

 ties common to both. It was the great discovery of Sir 



