THE FRAMEWORK OF THE BODY 41 



tendon, give the muscle its name biceps, which means "two-headed." The 

 lower end of the biceps is attached by another strong tendon to the upper 

 part of the radius. We speak of the relatively immovable anchorage at 

 one end of a muscle as its origin and of the attachment to the part that is 

 normally moved as its insertion. The origin of the biceps is at the shoulder, 

 and the insertion is on the radius. The movement that results from the 

 shortening of a muscle is known as its action. When the biceps alone 

 contracts, its action is to pull the radius upward, bending the arm at the 

 elbow. But in normal arm movements many other muscles cooperate with 

 the biceps, and the elbow may be either bent or lifted, depending upon 

 what other muscles take part in producing the action. 



Now let us examine the structure of the biceps in more detail. We find 

 that it is covered with a tough, smooth sheath of connective tissue, which 

 binds its parts firmly together and allows the muscle to slide and move 

 over other muscles without noticeable friction. We note also that the 

 muscle is thickest near the middle and tapers toward the ends. If now 

 we cut the biceps in two across the middle, the cut end will be seen to have 

 a structure similar to that shown in Fig. 3.9. It is evident at once that 

 the muscle is not entirely composed of a single kind of tissue. In fact, as 

 was previously pointed out, a skeletal muscle is an organ, in which various 

 tissues contribute toward the performance of a particular function — 

 contraction and the production of movement. 



Within the outermost sheath of connective tissue there are a number 

 of smaller compartments, or bundles (fasciculi), separated by connective 

 tissue walls. These fasciculi are in turn composed of still smaller bundles, 

 also surrounded by connective tissue. And each of the latter (Fig. 3.9, 

 enlarged detail), when "teased out" beneath the microscope, is found to 

 contain many elongated, closely packed muscle cells (or muscle fibers 

 as they are often called). Blood vessels and nerves ramify in a fine net- 

 work throughout the whole muscle, so that each muscle cell is in contact 

 with blood capillaries, is bathed in fluid lymph exuded from the capillaries, 

 and receives the endings of one or more motor nerve fibers. 



Each of the elongated muscle cells is somewhat spindle-shaped. Unlike 

 most cells it contains numerous nuclei, scattered along its length just 

 under the cell membrane. Microdissection reveals the presence within 

 the cell of many fine threadlike strands, the fibrils, which run from end 

 to end of the cell and are immersed in a more fluid portion of the cyto- 

 plasm. Each fibril is made up of alternating sections composed of different 

 materials, like repeated beads on a string; under the microscope these 

 appear as a regular succession of light and dark portions of the thread. 

 Since the cell is packed with fibrils and these are all precisely "dressed" 

 upon one another, the entire cell (muscle fiber) has a cross-banded or 

 "striated" appearance (Fig. 2.3). 



