THORAX (.ao;pat from aopho to leap, be cause in it the heart beats). " The habitation of the breathing parts." — That part of the human body destined to contain the lungs and heart, and by its movements to maintain the function of respiration.
Generally by the term thorax is understood a cavity set apart for the respiratory organs. Such a cavity, however, is not essential to respiration : a respiratory surface only is es sential. This must exist in every animal, whilst a separate thorax is found perfect only in mammalia.
The development of the respiratory surface may take place in three ways.
1st. Either towards the interior of the body, in the form of ramified or sacculated ca vities ; or, 2ndly. Towards the exterior, in the form of lamellated, ramified, pectinated, tufted, cili ated, or pinnated processes called " branchiev," in which Nature seems to have exhausted all imaginable varieties of form ; and, 3rdly. By a system of tubes ramified to ex treme fineness, either in an especial cavity or thorax, or in a cavity common to these organs and to others destined for the diges tive function.
The movements necessary to respiration, arc modified according to the form of the re spiratory apparatus and the nature of the medium to be respired, whether pure air or air contained in water.
In some of the lowest animals, the respira tory movements are the same as those of loco motion, as in the monad and other infilsorial animalcules. In all animals, even when the respiratory organs are contained in a true thoracic cavity, the frame-work serves other purposes besides that of drawing in and throwing out air ; it gives attachment to the largest muscles of the upper extremity, whe ther prehensile or locomotive. In man parti cularly, we find the respiratory muscles con tribute to such acts as coughing, sucking, sneezing, yawning, sighing, singing, vomiting, as well as the innumerable articulate sounds of language.
Distinct respiratory movements, as depen dent upon alternate contractions and dilata tions of a thoracic cavity, are most regular, or at least, they have been more noticed, in mamtniferous animals. The thoracic cavities of rnammiferous animals have much of the mechanism of respiration in common. They all possess a vertebral column or spine, and that peculiar frame-work of ribs, together with a sternum, so articulated together as to move in breathing.
There is likewise a great similarity of mus cular arrangement around the thoracic cavity; and consequently the respiratory movements closely resemble each other.
Although the boundaries of the thorax are generally the. parts which move in respiration, and these are generally composed of vertebrm ribs and sternum, yet some animals may have either all of these, or they may lack some of them, or, if present, they may not move in the breathing function. Frogs have a ster num, but no ribs ; serpents have ribs, but no sternum ; tortoises have ribs, vertebra?, and sternum in one mass, rigid and immoveable : the crocodile and lizard have perfect ribs, but their sternum is almost entirely cartilaginous; and, lastly, in man, the components of the thoracic cavity may have a mobility to com mand or exceed a space equal to the whole cavity allotted for the respiratory organs. The relative quantity of air which he can respire for the aeration of his blood is probably greater than in any other animal, and his movements are more under the control of his will, and are greatly influenced by mental emotions.