Il8 HISTOLOGY 



McGill, the deeply staining nodular thickenings of muscle fibers indicate 

 a normal form of contraction in which the fiber does not contract as a 

 whole, but a wave of contraction passes over it. In these contraction 

 nodes the diameter of the fiber becomes increased (Amer. Journ. Anat., 

 1909, vol. 9, pp. 493-545). The enlargement of such muscular tubes as 

 the vessels and intestine appears to be passive and due respectively to 

 the pressure of the blood or food within. After extreme contraction the 

 elastic tissue probably serves to dilate the tube to a certain size. 



Smooth muscle is nourished by capillary blood vessels which tend to 

 follow the course of the fibers, and it is innervated by slender branches of 

 the sympathetic nervous system. 



SKELETAL MUSCLE. 



The skeletal muscles develop primarily from the mesodermic somites, 

 which have been briefly described in a previous section (p. 39). The trans- 

 formation of a portion of each of these blocks of tissue into layers and masses 

 of skeletal muscle fibers has recently been reviewed by Williams, from whose 

 work Fig. 108 has been taken (Amer. Journ. Anat., 1910, vol. n, pp. 55- 

 100). In Fig. 108, A, the core of the somite has fused with the ventral 

 and medial walls of the mass, and the tissue thus formed is streaming 

 over the aorta and toward the notochord. This tissue, the sderotome, 

 becomes mesenchyma and gives rise to smooth muscle and various other 

 mesenchymal derivatives. In the part of the somite left in place, near 

 the groove x, the striated muscle fibers begin to develop. The cells here 

 elongate at right angles with the plane of the figure, and thus lengthwise 

 of the embryo. In an older stage (Fig. 108, B) these myoblasts have 

 multiplied and have begun to form a plate of muscle tissue, the myotome, 

 which extends ventrally as shown in C and D. The dorso-lateral wall 

 of the somite has meanwhile become a plate of tissue, the dermatome, which 

 with the myotome associated with it, is often called the dermo-myotome. , 

 The dermatome according to Bardeen produces only striated muscle 

 fibers; Williams finds that it forms only dermal connective tissue, and 

 others consider that it gives rise both to muscle and connective tissue. 

 The myotome is "entirely transformed into muscle fibers." The way 

 in which the myotomes extend ventrally and break up into the ventro- 

 lateral trunk and neck musculature, and the longitudinal fusion and 

 splitting of the dorsal part of the myotomes to produce the deep back 

 muscles of the trunk and neck, have been described by Warren Lewis 

 (Keibel and Mall, Human Embryology, 1910). The skeletal muscles 

 of the limbs have usually been described as arising from cells which have 

 migrated into the limbs from the ventral part of the myotomes. If this 

 takes place the cells which migrate become indistinguishable from mesen- 



