8 MUSCULAR TISSUE. 



its spherical cells and gristly intercellular substance in which 

 the cells are embedded. We can recognise three distinct kinds 

 of this cartilage — hyaline, fibrous, and elastic. The cells of 

 cartilage are placed in clear round 'spaces. Its varieties will 

 be pointed out when we come to more special parts. Suffice 

 now to say that it is found in both of the great divisions of the 

 animal kingdom, and may even constitute the entire skeleton of 

 some of the fish {Elasmohranehii). 



Osseous tissue, or bone, is hard and possesses a high degree 

 of rigidity, by the intercellular substance being hardened by 

 the deposition of carbonate and phosphate of lime, these salts 

 constituting about two-thirds of the weight of bone. The cells 

 (the bone-corpuscles) occupy spaces in this intercellular matter. 

 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 vaiscular 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 by 

 either cartilage or other connective tissue. 



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

 be detected in aU animals save the very lowest forms, which 

 are apparently nothing but undifferentiated protoplasm. 



3. Muscular Tissue is contractile : this power of contraction 

 is due, as has 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. Muscle-cells during move- 

 ment contract and expand. In some of the lower animals 

 we see 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 the 

 unstriated. 



