THE MUSCLES. 93 



colour, which, when separated and examined by the micro- 

 scope, exhibit an infinite number of cross streaks. All the 

 muscles known as voluntary ; the muscles of the eye-ball, 

 the internal ear, tongue, and palate, a great part of the 

 esophagous, the diaphragm, the sphincters, and those of the 

 trunk and extremities, belong to this class. The muscular 

 fibres of the heart are, however, faintly streaked, although 

 this organ occupies the centre of the system of organic life. 

 Cross-streaked muscles are found in many of the invertebrated 

 classes ; they are well seen in insects, Crustacea, and spiders, 

 and may be observed in the fibrous layer on the under side of 

 the umbrella of some medusae. In various animals, however, 

 possessing voluntary motions, the simple class of muscular fibres 

 is only observed; but it may be assumed as a general propo- 

 sition, subject, however, to some exceptions, that the streaked 

 muscles belong to the system of animal life, and the un- 

 streaked muscles to that of organic life, and that the former 

 are developed from the serous, the latter from the mucous 

 layer of the germinal membrane. 



[ 215. Much difference of opinion exists as to the cause of 

 the cross streaks observed in the fibrillse of voluntary muscles. 

 We refer to the works of Wagner, Valentin, Bowman, and 

 others for a statement of their various opinions, and proceed 

 to describe the appearance presented by a beautiful preparation 

 of a portion of one of the voluntary muscles of a pig in fluid now 

 before me, viewed with one-eighth of an inch object-glass, each 

 fibrilla appears to be composed of an investing membrane or 

 sarcolemma, from which transverse processes extend across the 

 tube, dividing it into a number of square discs ; these cells 

 or discs, it is presumed, are occupied by the primitive sub- 

 stance of the muscular tissue ; the discs are of a rectangular 

 form, and have the same dimensions in the long as in the trans- 

 verse diameter ; in those fibrillse which are stretched the discs 

 appear oblong, but in one unstretched fibril, which lies most 

 advantageously for observation, the diameters are equal ; the 

 ultimate fibre of muscular tissue therefore, appears to consist 

 of a longitudinal row of rectangular discs placed end to 

 end, as seen in Figs. 60 and 63. A number of fibrillae united 

 by delicate tissue form a primitive fasciculus, and many fas- 

 ciculi united by areolar tissue, make the common fibres of 

 muscle as seen by the naked eye. From this arrangement of 



