MECHANISM OF THE BONES AND THEIR ATTACHMENTS. 581 



traction induced by the will cannot be further increased by strong electrical 

 stimulation of the motor nerve. On the other hand, if the motor nerve be stimu- 

 lated so that a less powerful contraction results, the will is unable to strengthen 

 this contraction. The work performed by a muscle already fatigued is much more 

 exhausting than a greater amount of work performed when it has been rested. 

 Anemia gives rise to symptoms similar to those of fatigue, up to the point of 

 inability to contract; while an abundant supply of blood rapidly refreshes the 

 muscle. Fatigue of the legs, as after marching, hastens fatigue in the arms. Long- 

 continued watching and fasting favor fatigue. Massage exerts a favorable in- 

 fluence on fatigued muscles. 



If a muscle be completely exhausted by voluntary movement, and if, never- 

 theless, the will be allowed to act as if to excite a contraction, the muscle will 

 actually begin to contract again after a period of rest, until it becomes again 

 exhausted, and so on. Mosso and Brandis assume that involvement of the central 

 nervous system, including the psychic centers, is, in part, to betaken into account 

 in connection with fatigue in man. If a sensory stimulus be applied at the com- 

 mencement of a voluntary contraction, the movement will be intensified and 

 accelerated. 



Pathological. In rare cases a morbid increase in the liability to muscular 

 fatigue (myasthenia) has been observed without muscular atrophy or sensory or 

 reflex disturbances. 



MECHANISM OF THE BONES AND THEIR ATTACHMENTS. 



The bones exhibit in their spongy structure an internal architecture, consisting 

 of lamellae arranged for pressure and traction exactly in accordance with those 

 lines that would be constructed by graphic statics in the representation of the 

 forces in weighted beams of the same form. This architecture is, therefore, so 

 completely adapted to the function of bone that it combines the greatest capa- 

 bility as a supporting apparatus with the least expenditure of material. 



The joints are covered with a layer of cartilage, which moderates, by means 

 of its elasticity, the concussions communicated to the bones. The surface of the 

 articular cartilage is smooth, and thus permits the articular extremities to move 

 freely upon each other. At the outer boundary of the cartilage arises the capsule 

 of the joint, which encloses the cartilaginous extremities like a sac. The inner 

 surface of the capsule is lined by synovial membrane, which secretes the viscid, 

 slippery synovial fluid, and this facilitates considerably the free movement of 

 the surfaces. The outer surface of the capsule of the joint is covered with numer- 

 ous fibrous bands, which act partly as fortifying and partly as restraining liga- 

 ments. The bony processes also are included among the restraining contrivances, 

 for example the coronoid process of the ulna, which permits the forearm to be 

 flexed only to an acute angle; also the olecranon, which prevents hyperextension 

 at the elbow-joint. The continuous apposition of the articular surfaces is made 

 possible (i) by the adhesion of the smooth cartilaginous surfaces, covered with 

 synovial fluid and sliding on each other; (2) by the external capsular ligament; 

 and (3) by the elastic tension and the contraction of the muscles. 



The articular cavities must be regarded as cleft spaces, bounded by free con- 

 nective-tissue surfaces, and unprovided with endothelium. The articular carti- 

 lage and also the adjacent connective tissue are bare. The intima of the synovial 

 membrane does not consist of endothelium, but of protoplasmic cells provided 

 with processes, together with a fibrous interstitial substance. It is almost every- 

 where separated from the articular cavity by a thin layer of fibrillar tissue. 



The synovial membrane is composed of delicate bundles of connective tissue 

 intermixed with elastic fibers; it is provided on its inner surface in part with 

 folds containing fatty tissue and in part with villi containing blood-vessels. The 

 internal articular ligaments or cartilages are not lined by synovial membrane. 

 The points of attachment of the synovial membrane to the bones are termed 

 insertion-zones. 



The colorless, stringy, synovial fluid has an alkaline reaction and the compo- 

 sition of transudates. In addition, it contains a substance resembling mucin, as 

 well as albumin and traces of globulin, lecithin, cholesterin, fat, soaps, lutein, 

 and also salts. Excessive movement diminishes its amount and increases its density 

 and also the amount of mucin, but diminishes the amount of salts. 



With regard to the mode of movement, joints may be divided into the fol- 

 lowing classes : 



