nerve-fibre irritability is lost, and the muscle-fibre irritability retained; hence there is no condition of lost faradic and increased voltaic

irritability such as as characterises the degenerative reaction just described. Irritability is changed to the one form of stimulus just as to the other.

Between these two forms there are intermediate conditions. For instance, the nerves may present normal irritability, and the muscle the increased voltaic irritability and changed order of contraction met with in degeneration. Probably, in these cases, some nerve-fibres are degenerated, and lead to the increased irritability of some muscular fibres. In both nerve and muscle the character of the reaction is determined by the more irritable structures; hence it is normal in the nerve and altered in the muscle. This has been termed by Erb the "middle form of degenerative reaction. It would be more accurate to call it the "mixed form."

The various changes in irritability have been thought to indicate the existence, and various affection, of separate centres for the nutrition of the nerves and muscles, apart from, though acting through, the motor nerve-cells. Remembering that the nerves and muscles contain fibres which suffer in different degrees, the phenomena at present ascertained may all be explained on the simpler principle stated, without the assumption of these special centres, of the existence of which there is, indeed, no evidence.

Frequently the lowered irritability of degeneration in the nerves is preceded by a slight increase of irritability, very transient when the degeneration is acute, of longer duration when the degeneration is of the slower variety just noticed. In some morbid states, again, in which the change of nutrition in the cells and fibres is extremely slight, an increase may alone be discovered. I have found such an increase, for instance, in diseases regarded as functional, as paralysis agitans and chorea, and it is an interesting proof of the molecular changes which underlie, or result from," functional" maladies.

THE MUSCLES: THEIR ACTION AND PARALYSIS.

Disease of the motor nervous system is largely manifested by loss of muscular action. Individual muscles, as well as groups of muscles, are often separately affected. Hence it is desirable to consider the symptoms of the paralysis of the more important muscles, before we enter on the study of special diseases.

The symptoms of the palsy of any muscle is a loss of its normal action, and a knowledge of this action is essential for the comprehension of those symptoms. The two must therefore be considered together. They are positive and negative aspects of the same facts.

It may be well, at the same time, to mention the nerve by which each muscle is supplied.

The action of muscles is threefold. (1) By their tonic contraction they maintain the parts in a certain posture, independently of voluntary effort. By actual contraction they (2) produce certain movements, and also (3) oppose the action of other muscles by a feebler contraction, and thus steady the movement that results.

The complex way in which muscles act together, and modify each other's effect, renders the subject a very large one. Here, only an outline can be given of the more salient facts concerning the most important muscles. The reader who desires to pursue the subject further can do so in the Physiologie des Mouvements,' of Duchenne, to whose researches our present knowledge is largely due.

The DIAPHRAGM (phrenic nerves from the fourth and fifth cervical) although a double muscle with two nerves, habitually acts as a whole, the two halves contracting simultaneously and diminishing each lateral curve of the arch. The central tendon descends but little. The abdominal viscera are depressed, and the parietes protruded. If the hand is placed beneath the ribs, the descent of the viscera beneath the diaphragm can be felt. When the diaphragm contracts alone, as when the intercostals are paralysed, or the phrenic nerve is faradised, the ribs to which the muscle is attached are slightly raised during its action, and this elevation causes a slight expansion of the thorax. In ordinary breathing this expansion is lost in the action of the intercostals. In paralysis the inspiratory protrusion of the upper part of the abdomen is lost; it even recedes during inspiration instead of advancing, and a descent of the viscera can no longer be felt by the hand. There often results a remarkable alternation in the respiratory movements of the thorax and abdomen, the retraction of the one corresponding to the protrusion of the other.

STERNO-MASTOID (spinal accessory nerve), passing from the sternum and adjacent part of the clavicle to the mastoid process, inclines the head towards, and rotates the face from, the side on which the muscle contracts. Both muscles together support the head in the vertical position, and, if it is bent back, they bring it forwards into, but not beyond, this position. Paralysis of one muscle has no influence on the position of the head, and but little on its movements. Other muscles supplement the loss. This is no such thing as a “paralytic torticollis." In palsy of both muscles, the head can be balanced in the vertical position, but if it falls back, it can be brought forward only with great difficulty. Each sterno-mastoid is associated in action with the muscles of the other side; it is a "contra-lateral muscle." For instance, in asing the right arm, the head is turned to the right by the left sterno-mastoid. This association is sometimes reproduced in disease.

MUSCLES MOVING THE UPPER LIMBS.

MUSCLES MOVING THE SCAPULA AND SHOULDER-JOINT.-The Trapezius (special accessory, lower cervical and upper dorsal nerves) consists of three parts. The first, from the occipital bone to the outer end of the clavicle, is rarely used except in breathing (respiratory portion, Duchenne). The second part is that which passes from the lig. nuchæ, lowest cervical, and upper three dorsal spines, downwards and outwars to the acromion and outer part of the spine of the

scapula. The lowest part passes from the dorsal spines below the third, outwards and partly upwards, to the inner half and base of the spine of the scapula. The

FIG. 5.-Paralysis and wasting of tra pezius; alteration in contour of shoulder at rest.

FIG. 6.-Ditto, when the arms are raised (the right one being aided by another person).

second part is the chief elevator of the scapula and shoulder. With the third part it brings the scapula towards the spine, and puts the shoulder back. Both parts tend to rotate the scapula-acromion up, lower angle out. By this rotation the arm is carried above the horizontal level, to which the deltoid raises it. Paralysis of the highest part has little influence on the movement of the scapula, but causes a change in the contour of the neck (Fig. 5) especially conspicuous on deep inspiration. The change in the shape of the neck is very great, when the arms are raised, if the whole trapezius is wasted (Fig. 6). In palsy of the middle part, the elevation of the shoulder is imperfect; in that of the third part, the scapula is farther from the spine than normal. In palsy of all parts, the scapula becomes rotated (acromion down, inferior angle in) by the weight of the arm and the contraction of the opponents (Fig. 7). The rotation may mask the displacement outwards, due to the paralysis of the lowest part. If the clavicular part remains, there may be no rotation, but the scapula is lower than normal.

FIG. 7-Paralysis and wasting of deltoid and trap zius; rotation of scapula, from weight of arm, in consequence of the paralysis of the trapezius; progressive muscular atrophy.

The Rhomboids (fifth cervical nerve, by a branch that passes through the scalenus) first rotate the scapula on the outer angle, moving the lower angle inwards, and then move the whole scapula upwards and inwards. In strong elevation, they aid the trapezius, which prevents the rotation of the scapula. The rotatory action aids forcible depression of the raised arm. The muscles also fix the scapula for the action of the teres major. Their tone helps to keep the scapula against the thorax (opposing the pectoralis) and in its vertical position (opposing the serratus), and hence, in paralysis, the

edge of the scapula, at rest, stands out a little, leaving a furrow, and the scapula is slightly rotated (lower angle out). Movement is but little interfered with by the paralysis of the rhomboids, the most important effect being that the movement backwards of the raised arm by the teres and deltoid is feeble for want of the fixation of the scapula.

The Levator anguli scapulae (third cervical nerve) first rotates the scapula on the outer angle and then raises it. The muscle is usually paralysed with the trapezius, and then the scapula falls, but the special effect of its palsy is lost in that of the trapezius. If the levator is preserved, and the trapezius paralysed, there is great rotation of the scapula, which is, as it were, suspended by its inner angle (Fig. 7).

Serratus magnus (posterior thoracic nerve, from the fifth and sixth cervical) carries the scapula outwards, forwards, and slightly upwards, when the arm is put forwards. It tends to rotate the scapula on the inner angle (acromion up), the lower fibres most powerfully, but this rotation is prevented by the rhomboids and levator anguli. It does not raise the shoulder when the arm is hanging. It helps to fix the scapula when the posterior fibres of the deltoid move the raised arm back. If the scapula is fixed by the rhomboids, the serratus can act on the ribs, and aid forced inspiration. It has most inspiratory effect when the arms are elevated. In paralysis there may be little change in the position of the scapula at rest, but often there is slight rotation (lower angle in) from the unopposed tone of the rhomboids. When the arm is moved forwards by the anterior part of the deltoid, the scapula, no longer held against the thorax and moved forwards by the serratus, is rotated on its vertical axis by the action of the anterior part of the deltoid on the humerus, and of the middle part on the scapula. Thus the posterior edge recedes from the thorax, leaving a groove into which the hand can sometimes be placed (Fig. 8). The scapula is, at the same time, rotated; lower angle inwards and upwards. Elevation of the arm above the level of the shoulder is much weakened, but can be imperfectly effected by the middle part of the trapezius. Loss of the serratus weakens other movements, but does not abolish any. Inspiratory expansion of the thorax, when the arms are raised, is distinctly less on the paralysed side (Poore).

FIG. 8.-Paralysis of the serratus magnus; eversion and rotation of scapula when the arm is put forwards.

The Deltoid (circumflex nerve, from the four lowest cervical) abducts the humerus, the anterior and posterior fibres also moving the arm forwards and backwards respectively. The arm is raised least by the posterior, and most by the anterior fibres, but even the latter only elevate it to a right angle with the trunk. Hence, if raised by the anterior fibres, and then moved back by the posterior, it is at the same time depressed. Elevation above a right angle is by rotation of the scapula (trapezius and serratus). These muscles also fix the scapula for the deltoid, preventing the rotation (acromion down, lower angle

in) that the deltoid, acting alone, would cause. In paralysis, abduction of the arm, direct, forwards, and backwards is almost lost. All the abduction that

FIG. 9.-Paralysis of right deltoid; elevation of shoulder by trapezius on an attempt to raise the arm, which is slightly abducted by the supraspinatus.

remains is a trifling movement by the supraspinatus. An attempt to abduct results in rotation of the scapula and elevation of the shoulder (Fig. 9) from an excessive innervation of the associated trapezius and serratus, which, as we have seen, fix the scapula when the deltoid acts. Paralysis of single parts of the deltoid causes loss of the corresponding movements of the arm, but if the middle part only is paralysed, there is still a limited power of direct abduction by the conjoined contraction of the anterior and posterior parts, aided by the supraspinatus.

The Supraspinatus (suprascapular nerve) abducts the arm, moves it forwards, and rotates it in. It thus aids the deltoid. Isolated paralysis of the supraspinatus has little influence on movement or position, but if the deltoid is also paralysed, the head of the humerus falls away from the acromion far more than when the deltoid is paralysed alone.

The Infraspinatus (suprascapular nerve) rotates the humerus outwards, and in paralysis this movement is lost. A difficulty in writing is produced, the movement along the line being by this rotation of the humerus.

The Teres minor (circumflex nerve) has a similar action to the infraspinatus, and its palsy has a similar effect.

The Subscapularis (short subscapular nerve, from the fifth and sixth cervical) rotates the humerus in, and its paralysis lessens this movement.

The Latissimus dorsi (long subscapular nerve, from the brachial plexus—last four cervical nerves) lowers the raised arm, and puts it back; the upper part adducts the scapula, the lower depresses the shoulder by acting on the humerus, which it tends to drag out of the socket. It inclines the trunk a little, and both muscles together extend the trunk. In paralysis, forcible backward depression of the raised arm is lost, and the shoulder cannot be put back without being also raised (by the trapezius).

The Pectoralis major (anterior thoracic nerves from the brachial plexus) consists of two muscles, the action of the clavicular and sternal parts being different, The clavicular (which arises also from the highest part of the sternum), if the arm is hanging, brings the shoulder forwards and upwards, as if shivering; if the arm is raised, it is brought forwards and lowered to the horizontal position. The muscle is thus concerned, Duchenne says, in the "cut" of the swordsman and the benediction of the priest. The sternal portion lowers the raised arm from every position, and if the arm is hanging, it draws the shoulder down. Paralysis of the upper part has little effect on the movement of the arm, because the anterior fibres of the deltoid have the same action. It is easily recognised by making the patient put his arms in front of him and press the palms together. In paralysis of the lower part, even with the lattissimus, the raised arm can still be lowered accurately, by the weight of the arm and relaxa