518 OF THE NERVOUS SYSTEM AND ITS ACTIONS. 



issue the mandates by which the Will calls the muscles into action. It 

 must be borne in mind, however, that size is not by any means the only 

 indication of their comparative development. As we advance from the 

 lower to the higher Vertebrata, we observe a marked advance in the 

 complexity of the structure of the Cerebrum. Its surface becomes 

 marked by convolutions, that greatly increase the area over which 

 blood-vessels can enter it from the surrounding membranes ; and in pro- 

 portion to the increase in the number and depth of these, do we find an 

 increase in the thickness of the layer of gray matter, which is the source 

 of all the powers of the organ. The arrangement of the white or fibrous 

 tissue, which forms the interior of the mass, also increases in com- 

 plexity ; and as we ascend even from the lower Mammalia up to Man, 

 we trace a marked increase in the number of the fibres, which esta- 

 blish communication between different parts of the organ. It is, in fact, 

 not merely from the different parts of the gray matter which forms the 

 surface of the hemispheres, that these commissural fibres arise ; but also 

 from those isolated portions of vesicular substance, which are found in 

 different parts of their interior ; and an extremely complex system is 

 thus formed, which is still but very imperfectly understood. 



914. The most important group of commissural fibres, is that which 

 connects the Sensory with the Hemispheric Ganglia; that is, which 

 radiates from the Thalami Optici and Corpora Striata, to the stratum of 

 gray matter which forms the convoluted surface of the Cerebrum. 

 These fibres constitute, in fact, the principal part of the white sub- 

 stance of the brain ; the remainder being made up by the commissures 

 to be presently described, and by commissural fibres which (it is pro- 

 bable) connect the different parts of the Cerebral surface with each 

 other. It was formerly supposed (and is still maintained by many 

 Anatomists), that the radiating fibres which may be traced to the Cor- 

 pora Striata and Thalami Optici, pass through these bodies, and are 

 continuous with the Crura Cerebri and consequently with the sensory 

 and motor tracts of the Medulla Oblongata. But when the small size 

 of the Crura Cerebri is compared with the relatively enormous bulk of 

 the radiating fibres, it is obvious that the former can only contain but 

 a very small proportion of the latter ; and as no absolute * continuity 

 has been traced, it appears more conformable to Anatomical and Phy- 

 siological probability, to believe that the fibres of the Crura Cerebri 

 pass no further upwards than the Sensory Ganglia, and that the 

 radiating fibres take a fresh departure from these bodies, to pass to- 

 wards the surface of the Cerebrum. Thus, then, we should be led to 

 regard the Spinal Cord, Medulla Oblongata, and chain of Sensory 

 Ganglia, as precisely representing the entire Nervous System of Insects, 

 the character of whose action is essentially automatic; and to consider 

 the Cerebrum as an organ superadded to its summit, receiving all its 

 incitement to action from impressions transmitted to it through the 

 Sensory Ganglia, and carrying into effect its volitional determinations 

 and emotional impulses, not (as formerly supposed) by immediately ex- 

 citing muscular movements through nervous communications passing 

 direct from the convoluted surface of the Cerebrum, but by playing 

 downwards upon the Automatic apparatus by which its mandates are 





