SENSE.
The parts subservient to the animal functions, which, constituting the leading character of animals, have derived their name from that circumstance, afford to our observation a more clear and manifest gradation, from the most simple to the most compound structure, than any others in the animal economy.
In some of the most simple animals of the class vermes, particularly among what are called zoophytes, little or no dis tinction of similar parts (or structures) can be discerned, and we are unable to recognize any thing as a particular ner vous system, or even as a part of such a system. The power of sensation and vo luntary motion which these possess, as well as any other order or class of the ani mal kingdom, proves that the nervous matter must be uniformly spread through out their homogeneous substance. The almost transparent polypes (hydra,) which are often found with a body of an inch in length, and arms or tentacula of a proportionate size, appear to consist, when surveyed in the best light by the strongest magnifying powers, of nothing but a granular structure connected into a definite form by a gelatinous sub stance.
In many other vermes, and in insects, particular nerves can be distinguished, arising in general from a chord running the whole length of the body, and called the spinal marrow, the superior extre mity of which part, slightly enlarged, constitutes the brain. The latter organ, however, in both classes of cold and red blooded animals, and still more in those which have warm blood, has a much more complicated structure, and a far greater relative magnitude : all animals are, how ever, exceeded in both these points by the human subject, which, according to the ingenious observation of the learned Stimmerring, possesses by far the largest brain, in proportion to the size of the nerves which arise from it.
The vast superiority of man over all other animals in the faculties of the mind, which may be truly considered as a ge neral distinction of the human subject, led physiologists, at a very early period, to seek for some corresponding difference in the brains of man and animals. They
naturally investigated the subject, in the first instance, by comparing the propor tion which the mass of the brain bears to the whole body ; and the result of this comparison in the more common and do Mestic animals was so satisfactory, that they prosecuted the inquiry no farther, but laid down the general proposition, which has been universally received since the time of Aristotle, that man has the largest brain in proportion to his body. Some more modern physiologists, how ever, in following up this comparative view in a greater number of animals, dis covered several exceptions to the general position. They found that the propor tion of the brain to the body, in some birds, exceeds that of man ; and that se veral mammalia (some quadrumana, and some animals of the mouse kind) equal the human subject in this respect.
As these 'latter observations entirely overturned the conclusion which had been before generally admitted, Stim merring has furnished us with another point of comparison, that has hitherto held good in every instance : viz. that of the ratio, which the mass of the brain bears to the nerves arising from it.
Let us divide the brain into two parts; that which is immediaely connected with the sensorial extremities of the nerves, which receives their impressions, and is therefore devoted to the purposes of animal existence. The second division will include the rest of the brain, which may be considered as connecting the functions of the nerves with the facul ties of the mind. In proportion then as any animal possesses a larger share of the latter and more noble part ; that is, in proportion as the organ of reflection exceeds that of the external senses, may we expect to find the powers of the mind more vigorous and more clearly de veloped. In this point of view man is decidedly pre-eminent : here he excels all other animals tliat have hitherto been investigated.