THE RABBIT. 



85 



fibres are arranged in bundles (fasciculi), and tho con- 

 nective tissue around these bundles is called the perimysiiwi. 

 It is the muscle-fibres which possess the power of "con- 

 tracting " the connective tissue is purely passive, though 

 its elasticity may help in restoring the muscle to its original 

 shape when contraction ceases. The change of shape of 

 the whole muscle is simply the sum of the changes of shape 

 of its constituent fibres. 



It is highly probable that the change of shape of a 

 muscle-fibre is, at bottom, the same thing as the change of 

 shape -of an amreba's pseudopodium, only it is confined to a 

 particular direction. The exact structure of muscle is, 

 however, a very difficult matter to determine, and has long 

 baffled histologists. We can only give an outline of tho 

 facts here. 



2. Striped and Unstriped Muscle. Two main kinds 

 of muscle-fibre may be recognized plain or unstriped or 

 and striated or striped. The former is the 



simpler. 



An^unstriated muscle-fibre (fig. 49, A) is a spindle-shaped 

 cell (perhaps twenty times as long as broad), containing a^ 

 single nucleug. The proto- 

 plasm shews faint striations 

 (stripes) in the direction of 

 its length : these are very 

 different from the transverse 

 stiiations of striped muscle - 

 fibre. 



^striped muscle-fibre (fig. 

 50) may be several hundred 



Inrifr n<* nnv nn 



)ng as any 



Fig. 49. MUSCLE-FIBRES. 

 A, UNSTRIPED ; B, CARDIAC. 



A large oval nucleus is seen in the 

 centre of each. 



striated one ever is as much 



as an inch or more in length 



thus better deserving the 



name of "fibre." Tn^ 



The outermost layer of the protoplasm forms a firm 



nf 



jj^Jias many. 



tgctive sheath the_ sarcplemmaj. the nuclei lie just within 

 this^ The ~more liquid protoplasm within has a very 

 definite structure, consisting apparently of alternate lighter 



