Page:Encyclopædia Britannica, Ninth Edition, v. 1.djvu/890

Rh 838 ocess of process of deglutition or swallowing. When the food is rallow- received into the mouth, it is moistened by the secretion of the salivary and other buccal glands, and is broken down by the grinding action of the molar teeth. The buccinator muscles press it from between the gums and the cheek, and, along with the movements of the tongue, aid in col lecting it into a bolus on the surface of that organ. Dur ing the process of mastication the palato-glossi contract so as to close the fauces. When the bolus is sufficiently tritu rated and moistened, the palato-glossi relax, the tip of the tongue is pressed against the roof of the mouth, and by a heave backward of that organ the bolus is pressed through the posterior orifice of the mouth into the pharynx, where it is grasped by the superior constrictor muscles, and forced downwards by them and the other constrictor muscles into the oesophagus, and thence into the stomach. As both the nose and larynx open into the pharynx, the one imme diately above, the other immediately below the orifice of the mouth, it is of great importance that none of the food should enter into these chambers, and obstruct the respira tory passages. To guard against any accident of this kind, two valvular structures are provided, viz., the soft palate and the epiglottis, which, whilst leaving the orifices into their respective chambers open during breathing, may effectually close them when deglutition is being performed. As the bolus is being projected through the fauces into the pharynx, the soft palate and uvula are elevated and made tense, and at the same time the wall of the pharynx is brought in contact with it by the contraction of the palato- pharyngei ; the part of the pharynx into which the nose opens is thus temporarily shut off from that into which the mouth opens. If laughter, however, be excited at this time, the tension of the soft palate is destroyed, and part of the food may find its way upwards into the nose. The closure of the larynx by the epiglottis is due partly to the depression of that valve and partly to the elevation of the larynx. The backward heave of the tongue relaxes the ligaments which connect the front of the epiglottis to that organ, and enables the small epiglottidcan muscles to depress the valve. The elevation of the hyoid and larynx is due to the action of the mylo-hyoid, digastric, and genio-hyoid muscles, which pass from the lower jaw to the hyoid, and of the thyro-hyoid, which pass from the hyoid to the thyroid cartilage of the larynx; preliminary to their action, the lower jaw must be fixed, which is done by the closure of the mouth prior to the act of swallowing. The aperture of the larynx is thus brought into contact with the depressed epiglottis, which is adapted more exactly to the opening by a change in its form due to the projection of a cushion- like pad from its posterior surface. By these ingenious arrangements the adaptation of a single chamber to the very different functions of breathing and swallowing is effectually provided for. JOINTS AND MUSCLES OF THE UPPER LIMB. The upper limb is jointed to the trunk at the sterno- clavicular articulation. This is a diarthrodial joint: the bones are retained together by investing ligaments; a meniscus is interposed between the articular surfaces, so that the joint possesses two synovial membranes. A strong ligament, which checksAoo great upward movement, con nects the clavicle and first rib. The two bones of the shoulder girdle articulate with each other at the diarthrodial acromio -clavicular joint ; but, in addition, a strong ligament, which checks too great displacement of the bones, passes between the clavicle and coracoid. The movements of the upper limb on the trunk take place at the sterno-clavicular joint, and consist in the elevation, depression, and forward and backward movement of the shoulder. The movements at the acromio-clavicular joint occur when the scapula is [JOINTS AND MUSCLES rotated on the clavicle in the act of elevating the arm above the head. The muscles which cause these move ments are inserted into the bones of the shoulder girdle; the trapezius into the clavicle, acromion, and spine of the scapula; the rhomboid, levator anguli scapulae, and serratus magnus into the vertebral border of the scapula; the pectoralis minor into the coracoid; and the subclayius into the clavicle. Elevation of the entire shoulder, as in shrugging the shoulders, is due to the contraction of the trapezius, levator scapulae, and rhomboideus; depression partly to the weight of the limb and partly to the action of the subclavius and pectoralis minor; movement forward to the serratus and pectoralis; and backward to the trapezius and rhomboid. In rotation of the scapula on the clavicle, the inferior angle of the scapula is drawn forward by the serratus and lower fibres of trapezius, and backward by the levator scapulae, rhomboid, and lesser pectoral. The Shoulder Joint is a ball-and-socket joint, the ball Should being the head of the humerus, the socket the glenoid fossa J oill t- of the scapula. A large capsular ligament, which is pierced by the long tendon of the biceps muscle, and lined by a synovial membrane, encloses the articular ends of the two bones, and is so loose as to permit a range of movement greater than takes place in any other joint in the body. The muscles which cause these movements are inserted into the humerus; the supra-spinatus, infra- spinatus, and teres minor into the great tuberosity; the sub-scapularis into the small tuberosity; the latissirnus dorsi and teres major into the bottom of the bicipital groove; the pectoralis major into its anterior border; the coraco-brachialis into the inner aspect, and the deltoid, which forms the fleshy prominence of the shoulder, into the outer aspect of the shaft. Abduction and elevation or extension of the arm outwards at the shoulder joint are due to the supra-spinatus and deltoid; adduction or de pression, to the coraco-brachialis, latissimus, and teres major, assisted by the weight of the limb; movement for wards and elevation, to the anterior fibres of the deltoid, pectoralis, and subscapularis; backward movement to the latissimus and teres; rotation outwards to the infra-spinatus and teres minor; rotation inwards to the subscapularis, pectoralis, latissimus, and teres. A combination of abduc tion, movem ent forwards, adduction, and movement back wards, produces the movement of circumduction. Certain movements of the upper limb, however, take place not only at the shoulder joint, but between the two bones of the shoulder girdle ; for in elevating the arm, whilst the supra- spinatus and deltoid initiate the movement at the shoulder joint, the farther elevation, as in raising the arm above the head, takes place by the trapezius and serratus, which rotate the scapula and draw its inferior angle forward. The free range of movement of the human shoulder is one of its most striking characters, so that the arm can be moved in every direction through space, and its efficiency as an instrument of prehension is thus greatly increased. The movement of abduction, or extension, which elevates the arm in line with the axis of the scapula, is characteristi cally human, and a distinct F, G. 22. Outline sketch of human im- articular area is provided On merus. The articular area for com- ,, , - T i f plete extension lies to the nght of the the head of the humerus tor dotted line. (After Qoodtir.) this movement. The Elbow Joint is the articulation between the humerus, Elbow, radius, and ulna : the great sigrnoid cavity of the ulna is adapted to the trochlea of the humerus, and the cup of the radius to the capitellum. The joint is enclosed by a