THE MUSCULAR SYSTEM. 



233 



Fijr. 95. 



and by their mutual connection form a continuous system of parallel 

 tubes, thus dividing the tendinous fibrils or primitive fasciculi into 

 numerous larger or smaller 

 groups. Secondary fasciculi, 

 mostly of a polygonal, or per- 

 haps rounded or elongated 

 figure, and having a diameter 

 of 0-03-0-06 of a line, may be 

 very readily distinguished ; and 

 tertiary fasciculi, with poly- 

 gonal contours, of 0-1-0-05 of 

 a line, and more in diameter, 

 and bounded by rather stronger 

 dissepiments ; there are, also, 

 generally apparent, still larger 

 subdivisions, composed of nume- 

 rous tertiary fasciculi, and 

 which, being closely united in 

 very various numbers and 

 groups, by a common envelop 

 of lax connective tissue, constitute the tendon itself. The aponeuroses 

 are constituted either in the same way as the true tendons, and consist 

 of several layers of parallel, secondary fasciculi, disposed contiguously 

 in the same plane, or more resemble the fibrous membranes, and present 

 primary and secondary fasciculi decussating in two or more directions 

 (abdominal muscles, diaphragm). 



Fine elastic fibres (the so-termed nuclear fibres) occur in the secondary 

 fasciculi of all tendons, in various conditions of development : sometimes 

 as a series of slender fusiform cells connected by delicate processes ; 

 sometimes as fully-formed fibres of uniform thickness, or as isolated 

 fusiform cells. The arrangement of these elastic elements is uniform 

 throughout, and they run at regular distances, parallel to, and among 

 the fasciculi of connective tissue. Consequently, in the transverse 

 section of a tendon, the dark ends of the elastic fibres are apparent, 

 distributed, at constant distances of 0-007-0-008 of a line apart, over 

 the whole section. But, besides these stronger elastic filaments, mea- 

 suring from 0-0005-0-001 of a line, there exist in most, perhaps in all 

 tendons, extremely delicate fibrils of 0-0002-0-0004 of a line, connecting 

 the former in various directions, so that in reality there is, in every ten- 

 don, an elastic network, penetrating and entwining the fasciculi of con- 



FIG. 95. Transverse section of a tendon of the calf, magnified 20 diameters: a, secondary 

 fasciculus; 6, tertiary ; c, nuclear fibres not quite in transverse section, but appearing as 

 little streaks in the former ; rf, interstitial connective tissue. 



