nevertheless impressed so forcibly upon it, that, under certain circumstances, their vestiges remain, after the removal of the objects from before the eye.'

NOTES.

(A) I am not satisfied with any account which I have hitherto seen, of the function of the eyelids with respect to the tears. If I am not mistaken, the tears pass over the ball of the eye as low as the edge of the superior tarsus, which is so applied to the ball as not ordinarily to allow of their ready escape under it. As the lids cover the eye during sleep, and their fine inner edges meet, the whole of the ball is at this time readily preserved moist. But when the eyes are open, the front of the eye between the lids would not be moistened unless the upper tarsus occasionally descended with the fluid contained behind it. The fluid thus brought upon the front of the eye, trickles down by its gravity as far as the inferior tarsus, which also occasionally ascending, raises it somewhat.

* Gassendi's vita Peireskii, p. 175, sq. Hague, 1655, 4to. Franklin's Letters on Philosophical Subjects, at the end of his Expts. on Electricity. Lond. 1769, 4to. p. 469, sq.

Rob. War. Darwin's Experimenta nova de spectris s. imaginibus ocularibus, quæ objectis lucidioribus antea visis, in oculo clauso vel averso percipiuntur. Lugd. Bat. 1785, 4to.

Er. Darwin's Zoonomia, T. i.

C. Himly in the Biblioth. Ophthalmolog. T. i. P. ii. p. i.

+ The object of this firm application of the tarsi to the eye must be the exclusion of foreign matters from the orbit.

Winking thus preserves the front of the eye constantly moist during the waking state.

It may be also observed that when the tarsi approximate, as they drive before them the moisture of the front of the eye-ball, they quite inundate the puncta lachrymalia, by which circumstance the puncta are of course enabled to carry off a large quantity of the secretion, and ordinarily to prevent its overflow, which would occur at the centre of the lower tarsus. During sleep the puncta are not so copiously supplied, as they have only the same share of tears as the eye in general; and there is less occasion for it, because the removal of the stimulus of the air and light by the closure of the eyelids, lessens the secretion.

M. Majendie has found the matter of the tarsal or Meibomian glands to be not sebaceous but albuminous, and soluble in the tears; hence we discover why, during sleep, it accumulates on the tarsi ;-because its solvent, the tears, are not sufficiently abundant to remove it.

(B) In Albino animals, whether the rabbit, pigeon, or mouse, the sclerotic and chorioid are nearly transparent, the latter losing its blood after death, and the image formed upon the retina may be readily seen, without removing a portion of the sclerotic. From observations of this kind M. Majendie has found that whether the eye be presented to a neighbouring or distant object, the image upon the retina is equally distinct, and therefore all the explanations of this circumstance which have been hitherto given, founded on changes which can occur only during life, fall to the ground, whether founded on pressure of the ball by the recti muscles, motion of the crystalline, contraction of the crystalline or ciliary processes, &c. He also found that the escape of a little of the aqueous or

vitreous humour, or the total removal of the former or of the cornea, impaired the distinctness of the image; the total removal of the aqueous humour or of the crystalline also increased the size of the image; the removal of the humours prevented the formation of any image; an increase produced in the pupil by a circular incision of the iris produced an increase of the image.*

• Précis Elementaire de Physiologie, T. i. p. 61, sq.

SECT. XVIII.

OF THE VOLUNTARY MOTIONS.

283. WE have seen that the nerves perform two offices: (220) the one of feeling, the other of moving. The former we have already considered; we shall now say something with respect to the latter.

284. All the motions of the body may be divided into voluntary and involuntary.

The pulsation of the heart, and the peristaltic motion of the intestines and other viscera, are commonly adduced as instances of involuntary motion.

The action of by far the greater number of the other muscles is voluntary.

Respiration, sneezing, the tension of the membrana tympani, the action of the cremaster, are regarded by some as belonging to the former class; by others, to the latter; and by others, as of a mixed nature.

285. If this division is narrowly examined, it will be found embarrassed by so many difficulties, that the limits of each class cannot well be determined.

For, on the one hand, few functions can be termed truly involuntary, especially if we consider the connection of the imagination and passions with the will.

Again, on the other hand, there are few voluntary motions that may not be rendered involuntary by the force of habit, whose influence upon our animal motions is immense.

M

286. Of the latter description are those muscular motions which, although generally voluntary, take place, under certain circumstances, without the knowledge of the mind, or even in opposition to its endeavours.

Thus we wink involuntarily, if a friend suddenly approaches his finger to one of our eyes, though it does not come in contact: the ring finger generally bends if we bend the little finger.

We often unconsciously move our limbs even while sleeping soundly.

On the contrary, some muscles which are almost always obedient to the will, occasionally cease to be so: an instance of this exists in the difficulty which we experience in attempting to move the hand and foot of the same side in different directions; and in all those motions, which, although voluntary and perfectly easy if produced separately, are found very difficult if attempted together.*

287. Among those motions which are supposed to be perfectly involuntary, no one is free from exception, as far as I know, excepting the spasms of the uterus during labour.†

With respect to the motion of the heart, we have the indubitable testimony of Baynard and Cheyne, that they saw the famous English officer, who could stop the motion of his heart and arteries at pleasure.

There is no question that the pulsation of the heart and

* Consult Winslow in the Mém. de l'Ac. des Sciences de Paris, 1739. These are partly voluntary in some warm-blooded animals, as is shewn in birds when setting, which, if deprived of their eggs, are well known to lay others in succession.

Cheyne's Treatise on Nervous Diseases, p. 307, sq.