There are, indeed, some persons who are so independent of vision that the loss of the visual centre does not much impair their normal speech.
When, again, an injury comes to the auditory centre in the temporal region, we find the converse of the case just described; the defect is then called Auditory Aphasia.

The patient can not now speak or write words which he hears, and can not speak spontaneously in proportion as he is accustomed to depend upon his memories of the word sounds.

But in most cases he can still both speak and write printed or written words which he sees before him.
These cases may serve to give the reader an idea of the remarkable delicacy and complexity of the function of speech.

It becomes more evident when, instead of cases of gross lesion, which destroy a whole centre, or cut the connections between centres, we have disease of the brain which merely destroys a few cells in the gray matter here or there.

We then find partial loss of speech, such as is seen in patients who lack only certain classes of words; perhaps the verbs, or the conjunctions, or proper names, etc.; or in the patients who speak, but yet do not say what they mean; or, again, in persons who have two verbal series going on at once, one of which they can not control, and which they often attribute to an enemy inside them, in control of the vocal organs, or to a persecutor outside whose abuse they can not avoid hearing.