86 



ZOOLOGY. 



and darker transverse discs, giving the transversely striped 

 appearance. More minute examination seems to show that 

 the darker bands are composed of many little longitudinal 



rod-like structures (" sarcous 

 rods"). The precise nature 

 of these structures is still 

 so uncertain that we will 

 not go into further detail. 



There is no doubt that 

 striped muscle represents a 

 higher degree of differentia- 

 tion or specialization for the 

 work of contraction than 

 does unstriped. This is 

 shown, firstly, by the dis- 

 tribution of the two kinds in the body : all the muscles 

 attached to bones (skeletal muscles), which therefore have 

 hard work to do, and are concerned in~the Inore 



Fig. 50. PORTION OF A STKIPKD 

 MUSCLE-FIBRE. 



The nuclei are not shown. 



muscle-fibres. On the other hand, those which have slow, 

 , routine work to do (splanchnic muscles), as~tEe 



muscles of the intestinal wall,_which cause its squirming 

 rt peristaltic " motionTare unstriped. So, too^lt is roughTy 

 true~Eat^striped muscle is voluntary, or under the control 

 of the will, and unstriped, involuntary ; while a more 

 accurate distinction lies in the fact that~ unstriped muscle- 

 fibres tend to contract rhythmically quite independently of 

 any stimulus ( 10, below), while striped fibres do not. 



The most important examples of unstriped muscles are 

 those of the wall of the alimentary canal, and those of 

 the walls of blood-vessels, and the structure of these walls 

 is described in the next two sections. Unstriped muscles 

 are also found in the walls of some ducts (e.g. bile-duct and 

 ureter) and of the urinary bladder, and in the iris of the 

 eye (chap, x., 8). 



3. Structure of Intestinal Wall. The wall of almost the whole 

 of the alimentary canal may be divided into three main layers, in 

 order from within outwards the mucous, sub-mucous and muscular 

 coats. The layer of peritoneum on the outside may be counted as a 

 fourth (serous) layer. 



