The adaptation to conditions of middle moun-tains is directly connected with the increase of ven-tilation of lungs and strengthening of influence on thorax of respiratory muscles. In the study the fea-tures of a cross chest muscle in marals were inves-tigated. As material for studying of features of mor- phology of transverse thoracic muscle of the marals from the foothills of the Altai and Altai Region served. Thus the topography of a cross pectoral muscle of marals is generally similar to that in the muscles of other animals, but it also has its own features: fixed not only on the dorsal part of the cartilage, but also on sternal ends of the ribs, rein- forcing the impact on the chest. On the dock it is subdivided into 6 teeth in the interval from the 2-nd to the 7-8-th ribs. An increase in the length of its teeth is observed in caudal direction. The first prong is separated from the main part, and merges with it only in the places of separation from the sternum. At the base of each prong tendon mirror is formed in the form of rays. Linear dimensions of the muscle with age tend to increase, intensive growth of the body is marked at 6 months of age (puberty), namely the width of the muscle layer by 2.1 times; the length of muscle bundles: the first prong is at 1.68 %, up to the fifth rib in 2.13, and in the teeth of 7-8-th of cartilage - 1.9; the thickness is 2.44 and 2.57 times compared to newborn. Absolute mass of transverse pectoral muscle also increases with age, and the maximum value is noted in 6-month age, relative weight decreases with age. Relative incre-ment of the body in 6 months is 124.8 % and then to 10 years is reduced by 2.5 times.