28 ANATOMY OF 11 1 P: KABBJ r 



of (he former kind liavc well (Icfmcd myofibrils and rather in- 

 coiispicuioiis transverse bands, are usually somewhat small in 

 diameler, and contain a greater quantity than do the pale fibres of 

 a reddish, iron-rontaining substance (myoglobin) related to the 

 haemoglobin of blood. The pale fibres have less cytoplasm, more 

 cons|)ienous transverse striations, and usually greater diameter, 

 and (iu'ii- nuclei are more nearly confmed to the inner surface of 

 (he sarcolenuna. Kvd fibres contract more slowly than white but 

 aie more resistant to fatigue. The proportion of these two types 

 xai-ies in differenl muscles, and in many animals (as man) one 

 never preponderates so greatly as to make an ol)vious difference 

 in the aj)i)earance of the gross muscles. In the rabbit and many 

 other am'mals, however, some nuiscles (e.g. the semitendinosus 

 and the soleus) are definilely red and others (e.g. the adductor 

 magnus) are pale or white. 



The muscular substance of the heart differs from both stri- 

 ated and smooth muscle in beingcomi)osed of branched anastomosing 

 fibres, which apparently form a continuous network and which 

 have (heii- nuclei (HMitrally placed. Like striated nuiscle, it possesses 

 charact eristic (ransvense markings, but, like involuntary muscle, 

 it is under I he control of the autonomic nervous system. Certain 

 differentiated cardiac nuisc le rvWs, the Purkinje fibres, constitute 

 a conducting system (bundle of His, etc.) which regulates the 

 contraction of the chambers of the heart. 



In the gross, voluntary muscles present a longitudinal striation 

 which is roughly referred to as the direction of the fibres, and which 

 is of great value in identification. The striation is due to the 

 circumstance that the fibres are arranged in parallel groups or muscle 

 bundles, each of which is surrounded and separated from the adja- 

 cent bundles by a connective tissue enclosure called the 

 perimysium. 



Involuntary muscle is distinguished by its white or greyish 

 coloration and by its smooth or homogeneous appearance. It forms 

 characteristic layers in connection with visceral organs or with the 

 skin, and is thus much less individual than the voluntary nmscles 

 in its relations to particular pavts. It forms the muscular por- 

 tion (muscularis mucosae) of the mucous tunic of the alimentary 

 <-\'mal, and also a separate muscular tunic lying in the outer portion 



