LESSON& IN ZOOLOGY. 253 



are made, and which invests the bones, muscles, nerves, etc. 

 Sometimes these fibers form a network which serves as the basis 

 of the skin and the fatty tissue. 



Cartilage with spherical cells and a firm intercellular substance 

 is another variety of connective tissue. Sometimes the intercel- 

 lular substance contains a network of fibers, forming a fibro-car- 

 tilage, and there are numerous other intermediate forms. 



When calcareous matters are deposited in the intercellular 

 substance of cartilage, it becomes osseous tissue. This bony sub- 

 stance is traversed by numerous canals, through which nutrient 

 fluids may pass to the cells. Sometimes the bones grow in thick- 

 ness by the deposition of calcareous matter in the inner layers 

 of their investing membranes. 



The protoplasm of the cell has the power of contractility, but 

 some aggregations of cells possess this in a high degree, giving 

 rise to the muscular tissue, which is the chief agent in all the 

 movements of the body. When acting, these cells change their 

 form,becomingshorter and broader than when at rest, thus caus- 

 ing the muscle to contract. Two kinds of muscles are recognized, 

 the smooth and the striated. The smooth muscles are made 

 up of flat, spindle-shaped or band-shaped cells, and are somewhat 

 sluggish in their action. They are more common among the in- 

 vertebrata, but constitute the involuntary muscles of the verte- 

 brata. The striated muscles are made up of elongated cells or 

 fibers, across which there are minute lines giving them a striped 

 appearance. These muscles are more prompt and energetic in 

 their action than the smooth muscle, and are usually under the 

 conscious control of the animal. 



The nervous tissue is made up of cells and fibers, which differ 

 not only in form, but in structure and chemical composition. The 

 cells usually contain a nucleus and nucleolus with a fine granular 

 substance, and have one or more branches, one of which is con- 

 tinued into a nerve fiber. The fibers consist of an axis cylinder 

 or thread of grayish matter, covered by a sheath of whitish 

 fatty matter, which in turn is covered by a sheath of connective 

 tissue ; such fibers are called medullated fibers. Sometimes the 



