of the tongue, of the soft palate, of the larynx, of the pharynx and oesophagus, and of the lower outlet of the pelvis. The opposite class comprehends the heart, the stomach, the small and great intestines, and the bladder.

The collateral differences, which characterize either class, are, with exceptions afterwards to be adverted to, the following.

Of the muscles, which act when a nerve distributed through them is mechanically irritated, it may be remarked,

1. That they admit of being thrown into action by an effort of the will.

2. That with sufficient attention and resolution, their action may be refrained from.

3. That their action is attended with a conscious effort, and has reference to sensation.

? 4. That if divided, the separate parts retract instantaneously to a certain point, and subsequently undergo no further permanent shortening.

: 5. That when mechanically irritated, a single and momentary action alone ensues.

6. That they remain relaxed, unless excited by special impressions, both in the living body and before the loss of irritability after death.

7. That their action in the living body habitually results from an influence transmitted from the brain, or spinal chord through the nerves.

The exceptions to be made against this statement, if applied generally, are that the three first affections are not easily brought home to the muscular fibres of the œsophagus, or of the lower part of the pharynx; but it

E

50

Distribution of Muscles

deserves at the same time to be considered, that the lower part of the pharynx and the œsophagus are in the peculiar situation of parts employed upon one object alone instinctively and habitually on the occurrence of a local sensation; a condition which would soon reduce a strictly voluntary muscle to a state apparently removed from the controul of the will.

Muscles of the preceding class, if we except the fasciculi belonging to the pharynx and œsophagus, and urethra, are so disposed as to extend from one piece to another of the solid framework of the body: they enlarge or straighten the cavities of the trunk; they produce the phenomena of the voice; they close the excretory passages; they move the limbs upon the trunk; the whole frame upon the ground. Muscles of the following class are employed, like the exceptions in the preceding, as tunics to the hollow viscera, the cavities of which they diminish in their action, and thus serve to propel their contents. The oesophagus, indeed, appears to partake of the nature of both classes of muscles; when the nervi vagi are pinched, one sudden action ensues in its fibres, and presently after, a second of a slower character may be observed to take place.

Of the muscles which do not act upon the mechanical irritation of any nerve distributed through them, it may be remarked,

1. That the will cannot instantaneously or directly produce action in them.

2. That efforts of attention, with the resolution to abstain from their action, are insufficient to repress it.

3. That their action is not attended with a conscious effort, and seldom has reference to sensation.

4. That if divided, the retraction which follows is in most instances slow and gradual.

5. That if they are mechanically irritated, not one, but a series of actions ensues.

6. That their natural state in the absence of external impressions is not continued relaxation. When the heart and bowels are removed from the body of an animal immediately after its death, they continue for a time alternately to contract and to dilate.

7. That an impression transmitted through the nerves does not appear the usual stimulus to their action.

.

The exceptions to be found to these remarks, are more numerous than in the preceding class, and their consideration would lead me into details too minute for this part of the work, in which my object has been to convey a general notion only of muscular action. Let me conclude the present chapter by observing, that the leading distinction pointed out among different modes of irritability appears applicable to other textures, besides those which are strictly termed muscular. The iris acts when one of the two nerves distributed to it is mechanically irritated. And on the other hand, the calibre of arteries is not diminished when their nerves are pinched; and the uterus and the skin, it is probable, are equally insensible to this mode of excitement.

CHAPTER IV.

OF THE FORCES WHICH CIRCULATE THE BLOOD.

THE stream of florid blood, collected from innumerable vessels in the lungs, flows to every part of the body. The stream of black blood collected from the capillary vessels of the whole body, flows again into the lungs. The former passes through the left cavity of the heart, the latter through the right. The structure of the heart is muscular; its action gives motion to the blood, which by the operation of valves is confined to one direction.

But in order to understand the mechanism of the circulation, it is necessary not merely to examine the disposition of the muscular lacerti and of the valves in the heart, and the structure and properties of arteries and veins, but in addition to consider the nature and influence of the cavity in which the heart is placed, and of the di- • latable and elastic viscera with which it is surrounded.

The thorax of a skeleton is a hollow conoïd,, broad below, narrow above, where it is obliquely truncated: its axis is inclined obliquely upwards and backwards: it is composed of the dorsal vertebræ, the ribs, and the sternum. The twelve dorsal vertebræ form a column so bent as to be concave forwards, and which in reference to changes of figure in the chest may be considered as fixed. The twenty-four ribs are individually moveable upon the spine in every direction, but to a degree extremely limited. The seven uppermost, or the true.

ribs on either side, are let in by slips of cartilage into oval fossulæ along the side of the sternum, which they support. The five lower, or false ribs, are attached each to that above. The ribs and sternum are slight and fragile bones. In composition they derive strength from their convex form, and numerous and elastic joints.

An imaginary plane carried through the first dorsal vertebra, through both articular extremities of the first rib of either side, and through the upper part of the sternum, would slant obliquely downwards and forwards. By the movement of each first rib upon its spinal joint sufficiently to raise the upper margin of the sternum to the height of the first dorsal vertebra, the imaginary plane would become horizontal. In man this motion of the first rib is very limited; but it is obvious that in proportion as it takes place, the vertical distance of the sternum from the spine, or the depth of the chest becomes increased. The six lower true ribs move more freely upon their vertebral joints, and contribute to raise and carry forward the middle and lower part of the sternum for the same purpose.

By this provision all the muscles of the trunk, the lower attachment of which is to the ribs, are rendered capable of increasing the depth of the chest or its diameter from before backwards, and the opposite class of muscles, of diminishing the area of the chest in the same dimensions.

All the ribs, but the first, admit of a limited degree of rotation upon their vertebral and sternal joints. Nature distinguishes in the foetal state the fixed character of the first rib, by forming its cartilage of a common substance with the sternum, with which the cartilages of the suc