DEVELOPMENT OF THE MUSCLES 371 



If the fulcrum rests on the earth the power must either directly or indirectly 

 push from the earth or be attached to the earth either by gravity or otherwise if it 

 pulls toward the earth. If the power were attached to the weight no lever action 

 could be obtained. 



There are no levers of the II Class represented in the body. 



DEVELOPMENT OF THE MUSCLES. 



Both the cross-striated and smooth muscles, with the exception of a few that are 

 of ectodermal origin, arise from the mesoderm. The intrinsic muscles of the trunk 

 are derived from the myotomes while the muscles of the head and limbs differentiate 

 directly from the mesoderm. 



The Myotomic Muscles. The intrinsic muscles of the trunk which are derived 

 directly from the myotomes are conveniently treated in two groups, the deep 

 muscles of the back and the thoraco-abdominal muscles. 



The deep muscles of the back extend from the sacral to the occipital region and 

 vary much in length and size. They act chiefly on the vertebral column. The 

 shorter muscles, such as the Interspinales, Intertransversarii, the deeper layers of 

 the Multifidus, the Rotatores, Levatores costarum, Obliquus capitis inferior, 

 Obliquus capitis superior and Rectus capitis posterior minor which extend between 

 adjoining vertebrae, retain the primitive segmentation of the myotomes. Other 

 muscles, such as the Splenius capitis, Splenius cervicis, Sacrospinalis, Semispinalis, 

 Multifidus, Iliocostalis, Longissimus, Spinales, Semispinales, and Rectus capitis 

 posterior major, which extend over several vertebrse, are formed by the fusion of 

 successive myotomes and the splitting into longitudinal columns. 



The fascia lumbo-dorsalis develops between the true myotomic muscles and the 

 more superficial ones which migrate over the back such as the Trapezius, Rhom- 

 boideus, and Latissimus. 



The anterior vertebral muscles, the Longus colli, Longus capitis, Rectus capitis 

 anterior and Rectus capitis lateralis are derived from the ventral part of the cervical 

 myotomes as are probably also the Scaleni. 



The thoraco-abdominal muscles arise through the ventral extension of the 

 thoracic myotomes into the body wall. This process takes place coincident with the 

 ventral extension of the ribs. In the thoracic region the primitive myotomic 

 segments still persist as the intercostal muscles, but over the abdomen these ventral 

 myotomic processes fuse into a sheet which splits in various ways to form the 

 Rectus, the Obliquus externus and internus, and the Transversalis. Such muscles 

 as the Pectoralis major and minor and the Serratus anterior do not belong to the 

 above group. 



The Ventrolateral Muscles of the Neck. The intrinsic muscles of the tongue, the 

 Infrahyoid muscles and the diaphragm are derived from a more or less continuous 

 premuscle mass which extends on each side from the tongue into the lateral region 

 of the upper half of the neck and into it early extend the hypoglossal and branches 

 of the upper cervical nerves. The two halves which form the Infrahyoid muscles 

 and the diaphragm are at first widely separated from each other by the heart. 

 As the latter descends into the thorax the diaphragmatic portion of each lateral 

 mass is carried with its nerve down into the thorax and the laterally placed Infra- 

 hyoid muscles move toward the midventral line of the neck. 



Muscles of the Shoulder Girdle and Arm. The Trapezius and Sternocleidomas- 

 toideus arise from a common premuscle mass in the occipital region just caudal to 

 the last branchial arch; as the mass increases in size it spreads downward to the 

 shoulder girdle to which it later becomes attached. It also spreads backward and 

 downward to the spinous processes, gaining attachment at a still later period. 



