XXIX.J THE PHYSIOLOGY OF MIND. 21 



existence ; and as eyes are the most generally distributed It begins 

 of these in the animal creation, I think it most likely that P'^^j^^ ^ 

 sensation is nearly co-extensive with the possession of special 



SGllSG 



eyes. But this is incapable of proof : we have no criterion 

 whatever of its presence or absence. Sensation is in 

 itself, of course, perfectly inscrutable. It is utterly im- 

 possible that we can ever know how or why it is that the 

 flow of a current of a peculiar kind of energy along a 

 nerve to its ganglion should be accompanied by sensation. 

 But we might have expected to find sensation the peculiar 

 function of some particular kind of tissue, so that the pre- 

 sence of sensation might be inferred with certainty from 

 its presence, and the absence of sensation from its absence. 

 Such, however, is not the case. Some nerves and ganglia Sentient 

 are sensory, others are not so ; and the microscope, so far gentimt 

 as we yet know, shows no difference whatever between nerves 



, [. ^ histologi- 



the structure oi the two. cally alike. 



We thus see that the sensory function is not a primary Parallel 

 or fundamental endowment of the nervous system, but has m^e^t'of 

 been added to its original functions in the course of deve- organs 

 lopment. The history of the development of the nervous functions. 

 organs is parallel with the history of the development of 

 the nervous functions. The spinal cord, which is the 

 principal nervous organ of the insentient life, is developed 

 first in the embryo, and the sensory ganglia grow out of it. 

 The sensory ganglia ^ are situated within the skull, but are 

 distinct from the cerebrum, or true brain. Besides the 

 ganglia of the special senses, there is among them a pair of 

 ganglia called the thalami optici, which (notwithstanding 

 their name) are believed to be the nervous centre for the 

 sense of touch. In close proximity to the sensory ganglia 

 is another pair of ganglia called the corpora striata, which Corpora 

 are believed (though the subject is an obscure one) to be ^^^^^^ '• 

 the ganglionic centres for the nerves of motion, in so far as 



^ In order to guard against a probable misconception, it ought to be 

 stated that, so far as we know, the ganglia are not the seats of sensation 

 any more than the nerve-fibres. Sensation is produced when certain 

 ganglia receive a stimulus from their nerve-fibres ; but the ganglion is 

 insentient if acted on by pricking or in any other way except through its 

 fibres, and the fibres are insentient if they are cut off from their ganglia. 



