316 



THE MUSCULATURE 



more complex muscles dense connective-tissue septa, or intramuscular fasciae, serve to separate 

 different regions of the muscle from one another. In general there are groups of muscle fibre- 

 bimdles surrounded by a greater amount of connective tissue, or perimysium internum, than that 

 Burrounding the individual fibre-bundles, and the latter are surrounded by a denser connective 



Fig. 338. — Diagrammatic Outlines to Illustrate Various Types of Muscle Archi- 

 tecture AND THE Relations of the Main Nerve Branches to the Fibre-bundles of 

 the Muscle. 



a. Two segments of the rectus abdominis muscle of a small mammal, b. Portion of sheet-like 

 muscle with two nerve-branches and intramuscular nerve plexus, c. Typical quadrilateral 

 muscle with nerve passing across the muscle about midway betwefen the tendons, d and e. 

 Two triangular muscles with different types of innervation, f. Long ribbon-Uke muscle 

 with interdigitating fibre-bundles, g. Unipenniform muscle, h. Bipenniform muscle, 

 i. Typical fusiform muscle. The main intramuscular nerve-branches are distributed to the 

 fibre-bundles about midway between their origins and insertions. N. nerve. 



tissue than that surrounding the component muscle-fibres. The muscles arc surroiuidcd exter- 

 nally by a more or less dense sheet of connective tissue called the perimysium externum, or 

 epimysium, which is continuous with the connective tissue within the muscle, the i)erimysium 

 internum. In the following pages 'muscle fibre-bundle' is used to denote small groups of 

 musclo-fibres, 'fasciculus' to denote Inrge, more or less isolated, groups of fibre-bundles. 



Embryonic development of muscles. — The voluntary muscles are of nicsodermal origin. 

 The muscles of the trunk arise (iliicfly from the myotomes, those of the head and limbs chiefly 

 from the mesenchyme in these regions. New mus(t]e fibres are formed mainly before birth. 

 After birth, growth of muscles depends on growth of individual muscle fibres. 



