58 HUMAN PHYSIOLOGY. 



The deltoid is a thick, triangular muscle covering the shoulder-joint. 

 Arising from the outer third of the clavicle, the acromial process, and the 

 spine of the scapula, its fibers converge to be inserted into the humerus 

 just above its middle point. Its action is to elevate the arm through a right 

 angle. Owing to its point of insertion it acts as a lever of the third order, 

 but, notwithstanding the advantageous points of insertion, it acts at a con- 

 siderable disadvantage, owing to the obliquity of its direction. 



The biceps muscle, situated on the anterior aspect of the arm, arises from 

 the upper border of the glenoid fossa and the coracoid process, and is inserted 

 into the radius just beyond the elbow-joint. Its action is to flex and supinate 

 the forearm and to place it in the most favorable position for striking a blow. 

 When the forearm is fixed, it assists in flexing the arm, as in climbing. 



The triceps muscle, situated on the back of the arm, arises from the scapula 

 and the posterior surface of the humerus, and is inserted in the olecranon 

 process of the ulna. In its action it directly antagonizes the biceps, namely, 

 extending the forearm. In so doing it acts as a lever of the first order. The 

 short distance between the muscular insertion and the fulcrum causes it to 

 act at a great mechanical disadvantage, but there is a corresponding gain in 

 both speed and range of movement. The muscles of the forearm are 

 very numerous. Their action is to impart to the forearm and hand a 

 variety of movements, such as pronation, supination, flexion, extension, 

 rotation, etc. 



The pectoralis major and pectoralis minor muscles form the fleshy masses 

 of the breast. Arising from the inner half of the clavicle, the side of the 

 sternum, and the outer surfaces of the third, fourth, and fifth ribs anteriorly, 

 the muscle-fibers converge to be inserted into the humerus and coracoid 

 process. Their combined action is to adduct, flex and rotate the arm inward 

 and to draw the scapula downward and forward, movements necessary to the 

 folding of the arms across the chest. 



The rectus abdominis and the obliquus externus assist in forming the abdom- 

 inal walls. 



The glutei muscles are three in number, are arranged in layers, and form 

 the fleshy masses known as the buttocks. They arise from the side of the 

 pelvis and are attached to the femur in the neighborhood of the great trochan- 

 ter. Their action is to extend the hips, to raise the body from the stooping 

 position, and to assist in walking by firmly holding the pelvis on the thigh 

 while the opposite leg is advanced in the forward direction. 



The rectus femoris, with its associates, the rectus internus and rectus 

 externus and the crureus, forms the fleshy mass on the anterior surface of 

 the thigh. The former arises from the anterior part of the ilium, the latter 

 from the femur. Their common tendon, which is united to the patella, is 



