Page:Encyclopædia Britannica, Ninth Edition, v. 20.djvu/497

RESPIKATION 479 We must now explain how the respiratory muscles effect these movements. How the Inspiratory Movements are Produced. The Rib Movements. These are caused by the contraction of muscles which are fixed either to the central axis of the body (including under that term the head and vertebral column) or to some point rendered sufficiently stable for the pur- pose by the action of other adjuvant muscles. Thus the M. levatores costarum arise from the transverse processes of the 7th cervical and eleven upper dorsal vertebrae, and are attached to the ribs below in series ; the M. scaleni spring from the cervical vertebrae, and are attached to the anterior parts of the first and second ribs ; he M. sterno- cleido-mastoidei arise from the side and back of the skull, and are inserted into the upper part of the sternum and the clavicle ; the M. pectoralis minor arises from the coracoid process of the scapula, and is inserted into the anterior ends of some of the ribs ; the M. serratus posticus superior arises from certain of the cervical and dorsal vertebne, and is inserted into the posterior part of certain of the ribs ; the M. cervicalis ascendens (part of the M. erector spinae) arises from certain of the cervical vertebrae, and is inserted into the posterior part of certain ribs. The M. serratus magnus and the M. pectoralis major, which are affixed on the one hand to the upper arm and to the scapula respec- tively and on the other to the ribs and to the sternum respectively, may in certain elevated positions of the arm and shoulder act as in- spiratory muscles. When all these muscles contract, A the ribs are raised in the twofold way already de- scribed, some pulling up the anterior ends of the ribs, and others causing the arched ribs to rotate B about an axis passing through their vertebral and sternal joints. In addition to the muscles just enumerated the M. in- tercostales extern! are undoubtedly inspiratory muscles. Every ex- ternal intercostal muscular fibre between a pair of ribs must, when it contracts, of necessity raise both ribs, as is clearly shown by the "accompanying diagram (fig. 6). Here a'b' must be shorter than ab, for if BAa = x, then Fig. 6. + 2AB (B6-Aa) cos a: ; hence ab will be larger the smaller the angle x, for the cosine increases as the angle B diminishes. By a similar geometrical treatment of the question it may be shown that the in- tel'nal intercostal muscles when they contract must of Fig. 7. necessity depress both the ribs to which they are attached. If the angle BAc'=x (fig. 7), then hence c'd' will be larger the larger the angle x. The case, however, is not so clear with reference to the anterior Eortions of the internal intercostals which lie between the carti- iges ; for it is evident that these fibres have the same direction with regard to the sternum as an axis as the external intercostals have with regard to the vertebral column as an axis ; that is to say, the geometrical diagram in fig. 6 applies to the intercartilaginous internal intercostals as perfectly as it does to the interosseous parts of the external intercostals, the inference being that the inter- cartilaginous internal intercostals tend to elevate the pair of ribs between which they stretch. The geometrical argument is, how- ever, overborne by physiological experiment : Martin and Hartwell have observed in the dog and the cat that the internal intercostals throughout their whole extent contract (not synchronously) but alternately with the diaphragm ; hence we must conclude that their function throughout is not iuspiratory like that of the diaphragm, but expiratory. The Movements of the Diaphragm. The muscular fibres of the diaphragm are arranged in a radial manner, or more strictly speaking in a manner like the lines of longitude on a terrestrial globe. The central tendon of the diaphragm corresponds to the pole of such a globe. The contraction of the fibres is expended on straightening the longitudinal curves rather than on pulling down the central tendon to a lower level ; in fact the central tendon moves very little in ordinary respiration. How the Expiratory Movements are Produced. The action. of inspiration disturbs many organs from the position of rest into which gravity and their own physical properties have thrown them. The ribs and sternum are raised from the position of lowest level ; the elastic costal cartilages are twisted ; the elastic lungs are put upon the stretch ; the abdominal organs, themselves elastic, are compressed and thrust against the elastic walls of the belly, causing these to bulge outwards. In short the very act of inspira- tion stores up, as it were, in sundry ways the forces which make for expiration. As soon as the inspiratory muscles cease to act these forces come into play, and the position of rest or equilibrium is regained. It is very doubtful whether any special expiratory muscles are called into action during ordinary respiration. The internal intercos- tals may in man be exercised in ordinary expiration (al- though they are certainly not so exercised in the dog and the cat); but in laboured expiration many muscles assist in the expulsive effort. The muscles forming the belly walls contract and force the abdominal contents against the relaxed diaphragm in such a manner as to drive it farther and farther into the thorax. At the same time by their attachment to the lower edge of the thorax these- same muscles pull down the ribs and sternum. The M. triangu- laris sterni, which arises from the back or thoracic aspect of the sternum and lower costal cartilages and is inserted into the costal cartilages higher up, can obviously depress the ribs. So also can the M. serratus posticus inferior, which arises from the thick fascia of the loins and is in- serted into the last four ribs. So also can the M. quadratus lumborum, which springs from the pelvis and is attached to the last rib. Indeed there is hardly a muscle of the body but may be called into play during extremely laboured respiration, either because it acts on the chest, or because it serves to steady some part and give a better purchase for the action of direct respiratory muscles. Certain Abnormal Forms of Respiration. Coughing. There is first a deep inspiration followed by closure of the glottis. Then follows a violent expiratory effort which bursts open the glottis and drives the air out of the lungs in a blast which carries away any light irritating matter it may meet with. The act is commonly involuntary, but may be imitated exactly by a voluntary effort. Hawking, or Clearing the Throat. In this acj; a current of air is driven from the lungs and forced through the narrow space between the root of the tongue and the depressed soft palate. This action can only be caused voluntarily. Sneezing. There is first an inspiration which is often unusually rapid ; then follows a sudden expiration and the blast is directed through the nose. The glottis remains open all the time. The act is generally involuntary, but may be more or less successfully imitated by a voluntary effort. Snoring is caused by unusually steady and prolonged inspirations and expirations through the open mouth, the soft palate and uvula being set vibrating by the currents of air. Crying consists of short deep inspirations and prolonged expira- tions with the glottis partially closed. Long-continued crying leads to sobbing, in which sudden spasmodic contractions of the diaphragm cause siidden inspirations and inspiratory sounds generated in larynx and pharynx. Sighing is a sudden and prolpnged inspiration following an unusually long pause after the last expiration.