STRIATED OR VOLUNTARY MUSCLE. 



457 



ganglia, from which delicate twigs pass between the bundles of muscle-cells. The 

 mode of their ultimate termination is described in connection with nerve-endings 

 (page 1015). 



Development. — All muscular tissue in the higher types, with the exception of 

 that found within the sweat-glands and the iris,^ may be regarded practically as a 

 derivation of the mesoblast. Reference to Fig. 34 (page 29) recalls the division 

 of the mesoblast into the parietal and visceral layers, the latter, in conjunction with 

 the entoblast, constituting the splanchno-pleuric folds by the union of which the 

 gut-tube is formed. The subsequent differentiation of the visceral mesoblast contributes 

 the layers of the wall of the digestive canal outside the epithelial structures derived from 

 the entoblast ; in typical parts of the tube these layers are the submucous, muscular, 

 and serous coats. The muscular tunic consists of the unstriped involuntary variety, 

 the component fibre-cells representing specialized mesoblastic elements. 



Fig. 473. 



Mesothelium of serous coat 



Mesentery 



Differentiating muscular tissue 



Young connective tissue 

 Epithelium lining gut- 



Section of developing intestinal wall, showing earliest differentiation of involuntary muscular tissue from splanchnic 



mesoblast. X 200. 



The details of the development of the muscular tissue include condensation of 

 the young mesoblast produced by conspicuous proliferation and increase in the cells, 

 followed by their gradual elongation and conversion into spindle elements. These 

 are at first short, but become extended as the tissue assumes its fully developed 

 character. In localities in which the involuntary muscle occurs in sparingly dis- 

 tributed bundles and net-works the mesoblastic elements gradually assume the form 

 of spindle-cells which for a time are inconspicuous and difficult to distinguish from 

 ordinary young connective tissue. The formation of the muscular tissue within the 

 walls of blood-vessels is closely identified with the intramesodermic origin of the 

 vascular channels, the entire walls of which tubes are contributions of the middle 

 germinal layer. 



STRIATED OR VOLUNTARY MUSCLE. 



The striped muscular tissue constitutes the conspicuous masses known as the 

 " muscles" or " flesh" attached to the bony framework of the body. These organs 

 are also termed the skeletal muscles, and supply the active agents in moving the 

 passive levers represented by the bones in producing the movements of the animal. 



The muscles are usually elongated in form, and consist of aggregations of bundles 

 of the ultimate contractile elements, \\\& fibres, grouped into fasclaili ; upon the size 

 of the latter depends the texture of the muscles, coarse or fine, as distinguished in the 

 dissecting-room. In localities in which the fasciculi are of large size, as in the gluteus 



' Szili : Archiv fiir Ophthalmol., Bd. liii., 1902. 



