24 



HABIT AND INTELLIGENCE. 



[chap. 



Sensory 



ganglia 



developed 



out of 



spinal 



cord, 



and 



cerebrum 

 out of 

 sensory 

 ganglia. 



The 



cerebrum 

 is tbe 

 organ of 

 conscious- 

 ness. 



Largest in 

 the highest 

 animals. 



Cerebrum 

 not in 

 direct 

 connexion 

 with the 

 oi'gans of 

 external 

 life. 



Its 

 structure. 



The spinal cord ^ is the chief nervous centre of the in- 

 sentient life ; it is the first part of the nervous system that 

 makes its appearance in the development of the embryo. 

 Out of it are developed, by a process like budding, the 

 nervous centres of the sentient life, consisting of the sen- 

 sory ganglia, and the pair of motor ganglia called the 

 corpora striata, which, as already intimated, are in the 

 closest nervous connexion with the sensory ganglia. The 

 nervous system of insects contains no parts of higher 

 grade than these. But in the Vertebrata a distinct 

 organ appears, which is developed by budding out of the 

 sensory ganglia, as they are developed by budding out of 

 the spinal cord. This is the cerebrum, or true brain ; it 

 is also called the cerebral hemispheres, for it is a double 

 organ, though in man and the rest of the highest animals 

 the two hemispheres are in close contact. We have every 

 reason to believe that the cerebrum is the organ of con- 

 sciousness and mind, as distinguished from unconscious 

 instinct. In fishes, which are the lowest of the vertebrate 

 classes, it is very small in comparison with the sensory 

 ganglia, but it increases in size as intelligence increases in 

 ascending the animal scale, until in man and the rest of 

 the most highly organized animals the cerebrum is many 

 times larger than all the rest of the nervous centres put 

 together. 



The cerebral hemispheres are often called ganglia, but 

 this is not quite an accurate expression. Like the spinal 

 cord, they contain masses both of ganglionic cells and of 

 nerve-fibres ; but, unlike either the spinal cord or the 

 sensory ganglia, their nerve -fibres are not in direct con- 

 nexion with any of the organs of external life — with either 

 the muscles or the organs of sense. The cerebral hemi- 

 spheres consist of masses of fibres radiating upwards from 

 the sensory ganglia, and terminating in masses of gan- 

 glionic cells which are continuously spread out under the 

 bones of the skull ; and these ganglionic masses, both those, 

 of the two hemispheres and the various parts of the same 



1 Including the medulla oblongata, which, though within the skull, is 

 really part of the spinal cord. 



