16 rHYSIOLOGY OF MUSCLES AND NERVES. 



each other. Briicke, therefore, supposes that the muscle 

 substance is in itself homogeneous or uniform, but that 

 in it are inserted small particles which are of double- 

 refracting power. When these particles are massed in 

 large numbers, and are regularly arranged, they refract 

 the light doubly, so that the whole of that particular 

 part seems to refract doubly, while the intermediate 

 parts, since they contain few or none of the particles 

 in question, continue to refract simply. These latter 

 parts, however, when seen under ordinary unpolarised 

 light, so that it is impossible to judge of their powers 

 of double refraction, appear lighter, while the former 

 appear darker ; and so together they cause the striated 

 appearance of the muscle. 



3. In one of these muscle-fibres it is necessary to 

 distinguish the contained matter and the containing 

 pouch. The latter is called the muscle-fibre pouch, or 

 sarcolemvia. In it, especially after the addition of 

 acetic acid, which causes the whole fibre to swell and 

 become more transparent, a number of longish pointed 

 kernels (jiiiiclei) are seen, and similar kernels occur also 

 in parts within the muscle-fibre. To the ends of the 

 muscle-fibre, which are rounded and are very uniformly 

 enclosed by the pouch, which must therefore be re- 

 garded as a long closed sac, the white cords mentioned 

 above attach themselves, and these are completely 

 coalescent with the sarcolemma. 



They consist of strong slender threads of the nature 

 of the so-called connective tissue. As a considerable 

 number of muscle- fibres constitute the trunk of the 

 muscle, these threads also unite into cords which are 

 called the muscle-tendons. They are sometimes short, 

 sometimes long, thicker or thinner according to the 



