THE STRUCTURE OF THE MOTOR ORGANS 81 



striped muscular tissue, which is its essential part, but which is 

 supported by connective tissue, nourished by blood-vessels and 

 lymphatics, and has its activity governed by nerves; so that a 

 great variety of things go to form the complete organ. 



A loose sheath of areolar connective tissue, called the peri- 

 mysium, envelops each muscle, and from this partitions run 

 in and subdivide the belly into bundles or fasciculi which 

 run from tendon to tendon, or for the whole 

 length of the muscle when it has no tendons. 

 The coarseness or fineness of butcher's meat de- 

 pends upon the size of these primary fasciculi, 

 which differs in different muscles of the same 

 animal. These larger fasciculi are subdivided 

 by finer connective tissue membranes into smaller 

 ones, each of which consists of a certain number 

 of microscopic muscular fibers bound together by 

 very fine connective tissue and enveloped in a 

 close network of blood-vessels. Where a muscle S m a ii G part ' o7 a 

 tapers the fibers in the fasciculi become less nu- Sfied^h^wirSlfs 

 merous, and when a tendon is formed disappear al- cross-striationand 



, i , , i i , , a couple of nuclei. 



together, leaving little but the connective tissue. 



Histology of Skeletal Muscle. Each muscle-fiber is developed 

 from a single cell and so constitutes a single histological element. 

 In the adult form, however, a muscle-fiber differs from an ordinary 

 cell in that it contains several nuclei. Muscle-fibers vary greatly in 

 size; ranging in length from 1 up to 35 mm. (gV in. to J in.), and in 

 diameter from 0.034 to 0.055 mm. (yio to 4 i^ in.) . Each fiber con- 

 sists of a certain amount of muscle substance, the muscle plasma, 

 inclosed in a transparent connective tissue sheath, the sarco- 

 lemma. This latter structure serves not only to hold the semi- 

 fluid muscle plasma in place, but also to transmit the pull of the 

 contracting fiber to the point of attachment of the muscle. The 

 most striking characteristic of a fiber's appearance is the series 

 of alternating light and dim transverse bands of nearly equal 

 width with which it is marked, and from which its designation as 

 striped muscle is derived (Fig. 46). Under the high power of the 

 microscope the muscle plasma is seen to be made up of a number 

 of longitudinal fibrils, the sarcostyles, surrounded by a homogene- 

 ous medium, the sarcoplasm. 



