Upon referring to the ordinary operations of our own minds, volition appears to take place whenever we have reason to anticipate a certain degree of gratification or advantage from its exertion. We know by experience the prompt influence of the will upon our muscular frame-we are able to conjecture with more or less certainty the consequences of different voluntary actions: —and we will with a general or precise understanding of what the result will be, and in order to obtain it. A hungry person knows that the food he prepares to eat will gratify his appetite; a drowning person hopes that his cries will bring people to his assistance. But there are other instances in which conscious motives cannot be assigned for voluntary actions. The infant at the breast, or struggling when first plunged into water, employs efforts for its sustenance or preservation no less voluntary, than those of the famished prisoner when nourished by his daughter's piety, or of the drowning Roman when he exclaimed, "Help me, Cassius, or I sink." But in the infant the motive which leads to the voluntary effort is not a calculation of advantage, but a spontaneous tendency, a blind inclination, an Instinct.

Instinct appears to consist in a natural tendency to the performance of definite voluntary movements upon the occurrence of certain sensations or states of inward feeling. We have experience of its influence in our own persons at every period of life, and can analyse its nature both under circumstances where it does not admit of control, and in cases where we are capable of modifying or repressing its impulses.

The sudden start which is observed in nervous persons upon hearing an unexpected noise, is an instinctive

Instances of instinctive Action.

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action of the former kind. The same phenomenon is observed in animals, in which its final cause appears to be their instantaneous removal from uncertain danger.

The expression of the countenance depends upon the instinctive action of its muscles. There are some occasions on which we can resist the natural tendency to wear a smile or a frown upon the face; but there are others on which the practised efforts of a man of the world are insufficient to prevent his thus betraying the expression of transient feeling. The constraint, of which we are conscious as the effort is more or less successful, is like that commonly experienced upon refraining from voluntary actions that have become habitual. We see in the advantage derived to the intercourse of society from this universal language, the reason why nature has implanted so strong an instinctive tendency to its use. .. Breathing is another action to which we are led by instinct, in order that when the attention is distracted from this necessity of the body, it may yet sustain no interruption; and throughout life the influence of the instinctive tendency over respiration appears never to be diminished or habitually superseded.

The movements employed in the prehension of food are primarily instinctive, but are afterwards in a great degree performed upon reflection. Nevertheless, the contact of the food resting upon the root of the tongue always excites an instinctive act of deglutition, which we may suppose intended to ensure the protection of the respiratory organs.

Many writers oppose instinctive to voluntary actions, and suppose that in the former a principle different from volition directs the muscular effort. I am induced to

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Are instinctive Actions voluntary ?

prefer the account of instinct which I have given, as it seems to me both more simple and more consistent with observation:

Instinctive actions are attended with a conscious effort like those which we will upon reflection, but unlike the action of the heart or stomach, over which the will exerts no influence.

As we are able at our pleasure to modify the expression of the countenance, so can we at any time instantaneously assume the control of breathing, and suspend it or accelerate it. Or if we have breathed for a short time by a conscious effort of the will with measured inspirations, we can allow the function to return to its usual rate without any abrupt transition as from one mode of action to a different mode.

One nerve alone is employed for the transmission both of the instinctive impulse, and of the deliberate influence of the will. The diaphragm is rendered equally insensible to either stimulus, when the phrenic nerve is divided: and upon cutting through the portio dura of the seventh pair upon the face, I ascertained that the muscles which it supplies are paralysed for every mode of

action.

Among the various examples of instincts peculiar to animals, the industry of the bee furnishes a theme of common admiration. The principle, by which the bee is led to model its cell with mathematical precision, is probably the same with that which leads the infant to its first inspiration. The insect in all likelihood foresees no advantage from its labour; and the skill which it displays marks but the curious wisdom of its Creator, who moulded the living being for sensation and enjoy

Principles, which supersede Instinct.

215

ment, yet directed its blind propensities with perfect art and unerring accuracy to important ends.

The instincts of animals in some degree give place to other principles. The young of every species have a remarkable tendency to imitate the habits of those around them. The human infant quickly adopts something of the manner and expression of those who nurture it. In singing-birds the influence of this principle is so remarkable, that if brought up exclusively with birds belonging to another species, they learn the song of their companions instead of the notes proper to their kind.

Memory is shown to belong to animals on numerous occasions; as for instance, by their caution in shunning places where they have been disturbed, to which their instincts would lead them to return. To the same principle may be attributed the seeming wariness of old animals in a wild state. Instances of apparent contrivance in animals result probably from the recollection, and consist in the repetition of former fortunate accidents.

But whatever vestiges we observe of human faculties in animals,—a susceptibility of attachment and of aversion, appetites like those of man, and a seeming sagacity in providing for their wants, they yet exhibit nothing that can strictly be compared with human reason. Animals appear to share the same first impulses with human beings, but not having the power to reflect, or deliberate, e or to conceive the combination of means for the attain- 2010. ment of an end, they derive little advantage from experience, and want that capacity of indefinite improvement, which alone might vindicate for Man an exclusive claim to Immortality.

* Phil. Trans. vol. lxiii. p. 254.

A

SECTION 2.

Anatomy of the Brain and Spinal Chord in Man.

The central organs of the human nervous system are contained in the cranial cavity and vertebral canal. We trace in the external convexity of the bones of the head, in their sutural union, and in the different density of the three layers of which they for the most part consist, a provision to protect the brain against external injury. In the numerous joints of the spine, and in the limited movement allowed to each, we trace a provision for the equable distribution of pressure upon the contents of the vertebral canal during its necessary flexions.

Upon partially removing the cranial bones or the arches of the spine, the outermost of three membranes is brought into view. The dura mater is a thick fibrous membrane of great strength; which in the cranial cavity serves as a periosteum to its concave surface, but which in the vertebral canal, where it receives the name of. theca vertebralis, forms a flattened cylindrical tube of less diameter than the arch of bone, from the periosteum of which it is separated by ligamentous, cellular, and adipose texture. The want of adhesion of the theca vertebralis to the bones that contain it is another provision for the security of its contents. Across the cranial cavity the dura mater' throws membranous processes, which are interposed between the divisions of the brain, and serve to support its parts. In the tense edges of these septa, or in other parts of the dura mater, channels are formed termed sinuses, which serve as the venous trunks of the brain; and from their dense un