152 SPECIAL PHYSIOLOGY. 



LOCOMOTIVE ORGANS IN MAN. 



The parts of the body concerned in locomotion, are usually divided 

 into the passive and active organs of locomotion. The passive organs 

 consist of the bones, joints, ligaments, both fibrous and elastic, and 

 tendons ; whilst the active organs are the muscles. 



The Bones. 



The names of the bones, and their position in the body, are else- 

 where mentioned ; the microscopic structure of the osseous tissue has 

 also been explained.* The bones support and protect the soft parts 

 of the body, as well as give effect and precision to the actions of the 

 muscles : for these purposes, the bones are hard, somewhat elastic, 

 and resistant. The hardness and strength of bone are sufficiently well 

 known ; its elasticity is well exemplified in thin long bones, like the 

 ribs. The hollowness of the long bones, endows them with a greater 

 comparative strength than if the same weight of bony tissue had been 

 employed in the solid form ; for it is a well-known fact in physics, 

 that the same weight of material affords more resistance, both to 

 downward and lateral pressure or force, when arranged in a tubular 

 form, than it does when disposed in a solid cylindrical mass of equal 

 length. Independently of its strength, due to the grosser mechanical 

 form or distribution of its substance, the osseous tissue, owing to its 

 microscopical structure, is endowed with a remarkable -innate strength 

 and elasticity ; for the compact bone, it will be remembered, consists 

 of innumerable interlacing fibres, disposed in the form of numerous 

 concentric laminae, surrounding the minute branching Haversian ca- 

 nals, and themselves cemented together into one firm mass. More- 



* A few additional details may be here given concerning the structure of bone. 

 The laminae or lamellae, as they are called, are either concentrically disposed around 

 the Haversian canals, or they are placed parallel with the general surface of the 

 bone, or they are irregular, and fill up the general substance of the compact tissue. 

 The fibrous structure of these lamellae is now well established : when decalcified, by 

 soaking in an acid, they are seen to consist of transparent decussating fibres ar- 

 ranged in a compact reticulated manner. The lamellae are, moreover, frequently 

 perforated by bundles of other fibres, which serve, as it were, to connect them to- 

 gether: these perforating fibres, when decalcified, generally resemble white con- 

 nective tissue fibres, but some have been compared to elastic fibres. The lacunae of 

 bone, as shown after maceration in acid, contain each a nucleated cell, which pre- 

 sents an irregular outline like that of the lacuna itself, and is regarded as corre- 

 sponding with the so-called connective tissue corpuscles (Virchow). Besides the 

 cancelli of the cancellated or spongy tissue of bone, and the short and frequently 

 communicating Haversian canals of the compact tissue, other spaces are occasionally 

 found in the latter, especially in growing bone; they are produced by a process of 

 absorption, and when first formed are irregular in outline, frequently encroaching 

 on neighboring lamellae ; they are named Haversian spaces. The periosteum, which 

 covers the bones everywhere, except at the articular surfaces and on the smooth 

 grooves for tendons, consists of an outer layer composed of white fibrous tissue and 

 bloodvessels, and of an inner layer of elastic fibres. Besides the yellowish marrow, 

 which exists in the long bones, and is composed chiefly of adipose cells, there is in 

 the short bones, and in those of the cranium, a reddish and more transparent me- 

 dulla, which contains only a few fat-cells, but numerous minute spheroidal nucle- 

 ated cells, or proper marrow cells. 



