Into the composition of the vocal appa ratus we know that muscular, fibrous, and cartilaginous tissues enter; and, as these are altered by age, the mechanism which they constitute might a priori be expected to suffer similar modifications. The larynx of the infant is small and almost circular ; consequently the lips of the glottis and the superior ligaments are very short. This configuration, viewed in connection with the immaturity of the muscular tissue, accounts for the shrill wailing cry, which is the only vocal sound produced at this early period of human existence, and the only one required, since the quick instinct of maternal affection can interpret these simple notes into an eloquent language. No very appreciable alteration takes place in these parts at the time when speech is acquired, for this attainment has more con nection with the development and command of the muscles of the pharynx and mouth, as well as with the organ of intelligence, which enables the human being to discriminate sounds and to imitate them. Fortunately the oral and pha ryngeal muscles are some of the foremost in development, being required in suction and deglutition. A progressive change goes on in the larynx, though it is not very evident till the period of puberty in the male, when the thyroid cartilage is elongated, and with it the thyro arytenoid muscle. At this epoch occurs the moulting of the voice, or an accession of gravity in the tones, occasioned by the elongation of the parts just mentioned. The projection of the pomum Adami takes place at the same time. In the female larynx scarcely any change occurs, and the voice in consequence remains acute. We have already spoken of the facial bones and their cavities, parts which exercise a very decided influence on the sonorousness of the voice.
We must now hasten to the consideration of the parts employed in that other distin guishing function of animals, viz., sensation. There are two grand divisions of the organs of sensation, those which we understand, and those which we do not. The former consist of the various kinds of animal mechanism whereby the external causes of sensation are modified, the latter of the nervous substance intermediate to the external excitant, and that state of consciousness which we denominate sensation. We know that the eye collects rays of light and con centrates them on its internal surface, but are utterly ignorant of the changes which the re tina, the optic nerve, and the brain undergo in producing that condition of our sentient exis tence which we call vision. It is true that we are aware that certain states of these parts are incompatible with sight ; but why they are so is quite beyond our knowledge. We are, as it regards our acquaintance with the adaptation of nervous tissue to the production of sensation, in the same predicament as a man who watches the working of a steam-engine, and knows that a certain quantity of fuel, of water, of valvular compression, &c. is necessary to its motion,
hut has no idea of the laws of caloric, vapori zation, constitution of elastic fluids, &c. Our science demonstrates the fitness of the external and internal ear for receiving, propagating, multiplying, and diffusing vibrations, but why the contact with the auditory nerve produces sound, is an all but impossible inquiry; as well as the reason why the sensation may be absent when the organ is in „perfect order, and the nerve to all appearance unchanged ; or why the sensation may occur without vibrations, as in dreaming, and many nervous disorders. The same may be said of the skin ; it is well adapted for coming in contact with the points or superficies of bodies, but who can say why the nerves spread over it occasion certain feel ings ? These remarks are premised merely to shew that it must not excite surprise, if we are unable to point out completely the changes which age produces in the human body, corres pondently with the changes of its sentient fa culties.
It has already, in all probability, struck the mind of the reader, that the great develop ment of the cerebral system in the infant is inconsistent with the principle which we have been endeavouring to demonstrate, viz., that the growth of the human body consists essentially in the elevation of the organs, subservient to the animal functions, from a rudimentary state. The more we grow, the smaller is the proportion of the brain to the rest of the fabric. But it is no less true that the functions of the brain grow with our growth. How then are we to reconcile these opposite facts ? We must cer tainly discard the opinion, that the bulk of the organ is proportionate to its power, and exa mine the composition and the relations of its various parts to each other.
Limited as our knowledge is of the requisite conditions of nervous substance for its func tion, we are notwithstanding aware of two ex tremes of softness and hardness, which comprise those states of the tissue which are compatible with the exercise of its peculiar faculty. Patholo gists well know that ramollissement and indu ration of the brain may produce the same lesion of function, viz., abolition of sensation ; while it is equally well known that approximations to the same conditions will produce impairment of this faculty. Now in infancy the brain is ex tremely soft, almost pultaceous, while in old age it is extremely hard in comparison, and the similarity of the two ages in many respects, but particularly as it regards the functions of the nervous system, is matter of universal ob servation. It might then a priori be suspected that one of the changes in cerebral growth would be a tendency to a certain intermediate degree of consistence, and this is found actually to be the case.