514 Caroline McGill. 



The relaxed muscle fiber of the carotid of the ox is a long, spindle- 

 shaped structures, much thicker in the center than toward the poles. 

 Al the ends it may branch and as before mentioned does anastomose 

 with neighboring fibers. It is made up of a much elongated central 

 nucleus, Fig. 82 ; outside of the nucleus is a small area of reticular 

 protoplasm, and outside of this, forming, in most cases, the bulk of 

 the cell, is a thick layer of longitudinally running myofibrils, Figs. 

 10, 34 a. If any sarcoplasm exists between the myofibrilla? it is not 

 demonstrable by the ordinary stains, such as hsematoxylin, eosin, 

 etc. Just beneath the elastica interna many of the smooth muscle 

 cells show a large amount of reticular protoplasm around tlie nucleus. 

 In such fibers the myofibrillse are restricted to a thin peripheral layer, 

 Fig. 9. These fibers closely resemble those already described in the 

 muscularis mucosae of the small intestine of the pig. Fig. 13. 



2. Myofibrils. 



In adult smooth muscle, just as in development, two types of myo- 

 fibrillse occur : ( 1 ) very fine fibrillie, evidently corresponding to the 

 •elementary fibrils of Apathy, 1890 and 1891, and to the "Binnen- 

 fibrillen" of Ileidenhain, 1898 and 1900; and (2) coarse fibrilS 

 v;hich seem to correspond to the primitive fibrils of Apathy. The 

 latter, in some particulars, also resemble the "Grenzfibrillen" of Hei- 

 dcnhain, in others more closely the myofibrillae of Benda, 1902. 

 There are often in a single muscle cell fibrillse showing all gradations 

 in size from the coarse to the fine myofibrillae. In completely 

 extended muscle the individual myofibrillae are, throughout their 

 entire length, of comparatively even caliber. All of the myofibrillae 

 stain intensely with protoplasmic stains and with iron-haematoxylin. 



During early development in the pig and the chick all of the myo- 

 fibrillae begin as exceedingly coarse structures. These later break up, 

 probably by longitudinal splitting, into finer fibrillae. In the older 

 foetus there is undoubtedly some formation de novo of fine fibrillae. In 

 the adult muscle the coarse myofibrillae represent either persisting 

 embryonal structures, in which case they are usually entirely homo- 

 geneous. Fig. 13, 15, or they may be formed by the subsequent 

 adhesion or union of the fine myofibrillae into bundles. In this last 

 condition the finer fibrillae entering into their formation may, at 

 times, be demonstrated. 



