436 STRUCTURE OF MUSCLE. 



is syntonin ; and though the general rule is that the primitive bundles 

 shall run isolatedly and parallel to each other, in certain cases they anas- 

 tomose, Fig. 60. In its ultimate construction, the form of fibre may be 

 regarded as consisting of a series -of cells, as shown \nFig. 214, the di- 

 ameter of which varies according to the actual condition of the muscle, 

 whether it is in the contracted or relaxed state, but which may be taken, 

 on an average, at the Yoiy o"o f an i ncn - The cells are placed end to end, 

 the boundary walls upon the end presenting the appearance of a delicate 

 transverse line. Each cell consists of two portions, a central spot and 

 a pellucid border. The pellucid border is considered by Dr. Carpenter, 

 whose views of muscular structure we are here presenting, to be the cell 

 wall, the central space being the cavity of the cell filled with some* highly 

 refracting substance. Dr. Carpenter speaks of the central spot as dark ; 

 an inspection of the photograph, Fig. 214, proves that it may be light if 

 exactly in focus. When the fibril is in a relaxed state the longest axis 

 of each cell coincides with the length of the fibril, but when contraction 

 occurs this axis shortens, and a shortening of the entire fibril is the re- 

 sult. A number of these fibrils, placed side by side, constitute a fascicu- 

 lus ; indeed, there may be many hundreds of them thus bound together. 

 When such a fasciculus is forcibly ruptured it presents different appear- 

 ances, according as the ends or sides of its constituent cells have cohered 

 most strongly together. If the lateral cohesion is weakest, the fascicu- 

 lus tears into its constituent fibrils, as was shown in Fig. 214, but if the 

 end cohesion is the weakest, it will tear into discs or plates, as in Fig. 

 Fig. 218. 218. The fasciculus is thus a bundle of fibrils, its 



diameter varying very greatly, and being, in man, 

 from the -^ $ to the -g-^ of an inch ; in females it 

 is, on an average, smaller. Each fibril is a linear 

 series of coalesced cells. The cells, as they form 

 the fibril, lose their rounded and assume a rectan- 

 gular appearance, as shown at a, <z, Fig. 214. It 



Muscular fasciculi torn & , ./. -, i PI -i . i 



in discs. therefore appears that each fibril must have its own 



investing sheath, the representative of the walls of the little cells which 

 have coalesced, and this, though not usually admitted by anatomists, ap- 

 pears plainly in the photograph from which that figure is taken. In 

 length, muscular fasciculi vary fromtthe sixth of an inch to two feet. 

 The larger animals furnish some that are even much longer. The nor- 

 mal form is doubtless cylindrical, but this is constantly departed from, 

 each accommodating itself to the pressure of the adjacent ones. The 

 sarcolemma serves as a partition between its included fibrils and the cap- 

 illary blood-vessels and nerves, which imbed themselves in the rounded 

 angular spaces between adjacent bundles. The cross section of a por- 

 tion of muscle shows the manner in which the sarcolemma and the fibres 



