CH. VI.J 



INVOLUNTARY MUSCLE 



87 



Red Muscles. 



In many animals, such as the rabbit, and some fishes, most of the 

 muscles are pale, but some few (like the diaphragm, crureus, soleus, 

 semi-membranosus in the rabbit) are red. These muscles contract 

 more slowly than the pale muscles, and their red tint is due to hsemo- 

 globin contained within their contractile substance. 



In addition to these physiological distinctions, there are histo- 

 logical differences between them and ordinary striped muscle. These 

 histological differences are the following : 



1. Their muscular fibres are thinner. 



2. They have more sarcoplasm. 



3. Longitudinal striation is therefore more distinct. 



4. Transverse striation is more irregular than usual. 



5. Their nuclei are situated not only under the sarcolemma, but 

 also in the thickness of the fibre. 



6. The transverse loops of the capillary network are dilated into 

 little reservoirs, far beyond the size of ordinary capillaries. 



Cardiac Muscle. 



The muscular fibres of the heart, unlike those of most of the 

 involuntary muscles, are striated ; but although, in this respect, they 

 resemble the skeletal muscles, they have 

 distinguishing characteristics of their own. 

 The fibres which lie side by side are 

 united at frequent intervals by short 

 branches (fig. 113). The fibres are smaller 

 than those of the ordinary striated muscles, 

 and their transverse striation is less 

 distinct. No sarcolemma can be dis- 

 cerned. Each fibre has only one nucleus 

 which is situated in the middle of its 

 substance. At the junctions of the fibres 

 there is a certain amount of cementing 

 material, stainable by silver nitrate. This 

 is bridged across by fine fibrils from cell 

 to cell. 



Plain Muscle. 



FIG. 113. Muscular fibre-cells from 

 the heart. (E. A. Schafer.) 



Plain muscle forms the proper muscular coats (1.) of the digestive 

 canal from the middle of the oesophagus to the internal sphincter 

 ani ; (2.) of the ureters and urinary bladder ; (3.) of the trachea and 

 bronchi ; (4.) of the ducts of glands ; (5.) of the gall-bladder ; (6.) of 



