« MUSCULAR TISSUE. 



Numerous canals (Haversian canals) run through the bone, con- 

 taining blood - vessels and nerves. The calcareous matter is 

 arranged in concentric rings round these canals, which begin 

 in that highly vascular periosteal layer that circumscribes the 

 whole bone and open into long spaces, the marrow canals, in the 

 axes of the long bones. In all cases bony tissue is preceded b}' 

 either cartilage or other connective tissue. 



The two animal-life tissues are muscle and nerve. These can 

 be detected in all animals save the very lowest forms, which are 

 apparently nothing but undifferentiated protoplasm. 



3. Muscular Tissue is contractile : the power of contraction 

 is due, as has already been pointed out, to the protoplasm itself. 

 By differentiation of the protoplasm of certain cells and groups 

 of cells the power of contractility is brought to a higher state of 

 efficiency, and a tissue, the so-called muscular tissue, is formed 

 solely for movement. jMuscle-cells during movement contract 

 and expand. In some of the lower animals we fi.nd cells in 

 which only part of the cell is differentiated into a muscle fibre. 

 A stage further, and we find the whole cell becoming elongated 

 and converted into a definite muscle fibril. 



Of muscle there are two kinds, the striated and tlie 

 unstriated. 



The unstriated muscle is composed of flat, elongate, spindle- 

 shaped bodies, which contract slowly and remain in a con- 

 tracted state for some time. They seldom are more than ^-Jo- 

 of an inch in length. They form muscles over which the 

 animal has no control, and are thus called involuntary muscles. 

 This variety is prevalent in the lower animals, but is also found 

 in all high forms of life. Each such muscle-cell has a distinct 

 nucleus. 



Striated or voluntary muscle consists of multinucleated masses 

 called primitive bundles. It is composed of long cylindrical 

 fibres, about ttdtt of an inch in diameter in mammalian muscle. 

 Most or all of the cell protoplasm is converted into a cross- 

 striped substance, due to alternate double and single refractive 



