12 



HISTOLOGY. 



(neuroglia) derived from the ectoderm, but lacking entirely in 

 nervous properties. These glia cells are extensively branched, 

 their branches running between fibres and cell bodies (Fig. 9, C~). 

 Muscular tissue is the special contractile element in the 

 body. It is of two kinds, different in origin, structure, and ac- 

 tion. The mesenchyme gives rise to the smooth, 

 muscle. This consists of long spindle-shaped cells, 

 each usually containing a single nucleus, and being 

 marked with fine longitudinal lines. These cells may 

 occur singly, or may be arranged in small bundles 

 or thin sheets ; and in all cases they are not under 

 control of the will, a fact that gives rise to the name, 

 involuntary muscles, often applied to them. Smooth 

 muscular tissue is slow in its action. 

 Striped muscular tissue, on the 

 other hand, is derived from the mes- 

 othelium by modification from the 

 muscle plates, to be described later. 

 It occurs usually in larger masses 

 than does the mesenchymatous 

 muscular tissue, and is (except in 

 the case of the heart) under control 

 of the will. This striped tissue consists either 

 of separate cells (heart muscles) or of usually 

 long cylindrical, so-called primitive fibres, each 

 of which contains several nuclei ; i.e., is syn- 

 cytial. In these primitive fibres the bulk of 

 the protoplasm has been altered into a strongly 

 contractile substance marked with fine trans- 

 verse lines. Around each fibre is a struc- 

 tureless envelope, the sarcolemma ; while the 

 fibres are bound together into muscle bun- 

 dles by means of connective tissue envelopes 

 (perimysium) bearing nerves and blood-ves- 

 sels, and continuous with the tendons and 

 fascia by which the muscles are attached to 

 other structures. The nuclei are oval, with 

 their long axes parallel to the direction of 



FIG. it. 



Smooth 

 muscle 

 fibres. 



FIG. 12. Cross 

 striped muscle fibres 

 showing the nuclei 

 () and the sarco- 

 lemma, s, where the 

 muscle fibre is torn. 

 From Hertwig, after 

 Gegenbaur. 



