The Muscles 61 



88. Muscles of the Shoulder and Arm. The arm is raised 

 from the side by a large triangular muscle on the shoulder, 

 the deltoid, so called from its resemblance to the Greek 

 letter delta, A. The biceps, or two-headed muscle, forms a 

 large part of the fleshy mass in front of the arm. Its use 

 is to bend the forearm on the arm, an act familiarly known 

 as " trying your muscle." Its direct antagonist is the 

 three-headed muscle, called the triceps. This forms the 

 fleshy mass on the back of the arm, its use being to draw 

 the flexed forearm into a right line. 



On the back and outside of the forearm are the extensors, 

 which straighten the wrist, the hand, and the fingers. On 

 the front and inside of the forearm are the flexors, which 

 bend the hand, the wrist, and the fingers. If these muscles 

 are worked vigorously, their tendons can be readily seen 

 and felt under the skin. 



89, Muscles of the Lower Extremities. Passing to the 

 lower extremities, the thigh muscles are the largest and 

 the most powerful in the body. In front a great, four- 

 headed muscle, quadriceps extensor, unites into a single tendon 

 in which the knee-cap is set, and serves to straighten the 

 knee, or when rising from a sitting posture helps elevate 

 the body. On the back of the thigh are several large mus- 

 cles which bend the knee, and whose tendons, known as 

 the " hamstrings," are readily felt just behind the knee. 



On the back of the leg the most important muscles, 

 forming what is known as the calf, are the gastrocnemius 

 and the soleus. The first forms the largest part of the 

 calf. The soleus, so named from its fancied resemblance 

 to a sole-fish, is a muscle of broad, flattened shape, lying 

 beneath the gastrocnemius. 



The tendons of these two muscles unite to form the 

 tendon of Achilles, so called because that hero is said to have 



