MUSCULAR TISSUE. 1 03 



In muscular tissue thus hardened, the primitive fibrillse 

 are loosened from one another, and in some parts of the 

 specimen are usually more or less separated. The con- 

 tractile substance, moreover, usually shrinks away some- 

 what from the sarcolemma, which then appears in the 

 transverse section as a delicate ring around the fibre. 



Blood-vessels are seen in the above preparation, but 

 they may be much better demonstrated in longitudinal 

 sections of a muscle whose vessels have been injected. 



I. b. INVOLUNTARY STRIATED OR HEART-MUSCLE. 



In mammalia, the heart-muscle differs in several 

 important structural features from the voluntary 

 muscle. 



The contractile substance has essentially the same 

 structure as the latter, but, instead of being arranged 

 in the form of elongated, unbranched cylinders or 

 fibres, without distinct cell-structure, in the heart- 

 muscle the fibres send off at frequent intervals short, 

 narrow processes, which join neighboring fibres, 

 forming a narrow- and long-meshed net. Further, 

 the fibres which, owing to the numerous anasto- 

 moses, are very irregular in form, are made up of 

 distinct segments or cells, each segment being 

 cemented at the ends to its neighbors, and fur- 

 nished with a flat, elongated, ovoidal, or often 

 rectangular nucleus. In the vicinity of the nuclei 

 we usually see a certain amount of granular material 

 or pigment particles. The nuclei instead of lying, 

 as in the voluntary muscles, at the surface of the 



