FINER STRUCTURE OF MUSCLES 315 



to the four parts or muscles which constitute the quadriceps as a whole. In ad- 

 dition to these comparatively simple compound muscles there are others in which 

 the various coitiponent fasciculi and the tendons of origin and insertion are numer- 

 ous and complexly interrelated. The intrinsic muscles of the back offer good 

 illustrations of muscles of this nature. 



In addition to muscles with distinct regions of origin and insertion, there are a 

 few voluntary muscles which surround hollow viscera or their orifices and have a 

 circular or tube-like form (sphincter muscles, voluntary muscles of the oesophagus, 

 etc.). 



Number of muscles. — A logical constancy does not appear always to have been 

 followed in the commonly accepted division of the musculature into muscles indi- 

 vidually designated. Most of the muscles are symmetrically placed in pairs, one 

 on each side of the body. Authors not only vary in the extent to which they carry 

 the subdivisions of the musculature on each side of the body into individual 

 muscles, but also in describing muscles placed near the median line either as single 

 muscles with bilateral halves or as paired muscles. In addition some muscles are 

 not constantly present, and there are differences of opinion as to which of these less 

 constant muscles should be classed with the normal musculature. The BNA 

 recognises 347 paired and two unpaired skeletal muscles, and 47 paired and two 

 unpaired muscles belonging to the visceral system and organs of special sense. 

 Of the skeletal muscles the head has 25 paired and one unpaired; the neck 16 

 paired; the back 112 paired; the thorax 52 paired, one unpaired; the abdomen and 

 pelvis 8 paired; the upper extremity, 52 paired; the lower extremity, 62 paired 

 (Eisler). 



Finer structure of muscles. — While no attempt can be made here to describe in detail the 

 finer microscopic features of muscle structure, some of the more general features of muscle 

 architecture may be briefly mentioned. 



The contractile cells of voluntary muscle are long, slender, multinucleated 'fibres,' the pro- 

 toplasm of which exhibits both cross and longitudinal striation. The longitudinal striation 

 is due to the presence of fibrils situated in the sarcoplasma. The cross striation is due to alter- 

 nate segments of singly and doubly refracting substance in these fibrils. The length of these 

 fibres in the human body varies from a few millimetres to sixteen centimetres or more, and the 

 thickness from ten to eighty microns. Each muscle-fibre is surrounded by an especially differ- 

 entiated sheath, the sarcolemma. Outside of this is a layer of delicate connective tissue, the 

 perimysium internum or endomysium, the fibres of which are in part inserted into the sarco- 

 lemma. This connective tissue, which is especially developed at the ends of the fibres, serves to 

 attach them either directly to the structures on which the muscle acts or to the skeletal frame- 

 work of the muscle. 



In the simplest mammalian muscles the muscle-fibres take a parallel course from tendon to 

 tendon, and are not definitely bound into secondary groups. An example may be seen in fig. 

 338, a, which represents two segments of the rectus abdominis muscle of a mouse. More often, 

 however, the individual fibres do not run the entire distance from tendon to tendon, but instead 

 they interdigitate, and the interdigitating fibres are bound up into secondary and tertiary 

 anastomosing fibre-bundles by connective tissue, in which there is usually a considerable amount 

 of elastic tissue. Fig. 338, b, represents diagrammatically this interdigitation of fibre-bundles as 

 seen in the abdominal musculature of one of the larger mammals. 



In most of the flat muscles of the body the fibre-bundles either take a nearly parallel course 

 from tendon to tendon or they converge from the tendon of origin toward the tendon of insertion 

 (see fig. 338, c-e). The greater the distance from tendon to tendon, the more marked is the 

 interdigitation of the constituent fibre-bundles. 



In elongated muscles the tendons of origin and insertion may either arise near the extremities 

 of the muscle or may extend for a considerable distance on the surface or within the substance of 

 the muscle. In the former case the belly of the muscle is composed of bundles of interdigitating 

 fibres which take a course parallel with the long axis of the muscle. This is shown diagrammatic- 

 ally in fig. 338, f. An example may be seen in the sartorius muscle of the thigh (fig. 411). 

 Wiaen the tendons extend far on the surface or within the substance of the muscle, the con- 

 stituent fibre-bundles take a course oblique to the long axis of the muscle. When they take a 

 course from a tendon of origin on one side toward a tendon of insertion on the other, the muscle 

 is called unipenniform (see fig. 338, g, and the extensor digitorum longus, fig. 415). In other 

 instances the fibre-bundles converge f'-om two sides toward a central tendon. Such a muscle 

 is called bipenniform (see fig. 338, h, and the flexor hallucis longus, fig. 416). When there 

 are several tendons in the muscle between which the fibre-bundles run obliquely, the muscle 

 is called multipenniform. In fusiform muscles the tendons usually either embrace the ex- 

 tremity of the muscle like a hollow cone, or they extend far on the surface or within the sub- 

 stance of the muscle. In such muscles the fibre-bundles take a curved course from one tendon 

 to the other. The bundles which arise highest on one tendon are inserted highest on the other, 

 and the fibre-bundles of lowest origin have the lowest insertion. This structure is diagram- 

 maticallv shown in fig. 338, i. A good example may be found in the rectus femoris muscle 

 (fig. 411). 



Many other arrangements of the fibre-bundles are found, and the arrangements here shown 

 may be variously combined. In most muscles the architectm-e is decidedly complex. In the 



