130 COMPARATIVE MORPHOLOGY OF VERTEBRATES. 
higher mammals, where nerve supply still shows the original history. 
In the ichthyopsida the trunk muscles clearly show their myotomic 
origin, but even here there are tendencies to division and specialization. 
The ventral muscles on either side of the body cavity of the amphibia 
(fig. 140) are divided into a lateral oblique and a medial rectus sys- 
tem, the rectus muscles of the two sides being separated by the linea 
alba already referred to. The rectus muscles, in turn, become divided 
into successive groups, a rectus abdominis in the abdominal region, 
extending from the pelvis to the sternum; a sternohyoid from the 
Fic. 138.—Diagram of muscle segments in head of embryo vertebrate, based upon a 
shark, after Neal. The anterior myotomes tend to divide into dorsal and ventral moieties; 
persistent myotomes lined, transient with broken lines; central nervous system dotted, 
nerves black. a, premandibular somite; ab, abducens; nerve, hyp, hypoglossal musculature; 
hymn, hypoglossal nerves; om, oculomotor nerve; s#, spiracle; 1-6, first six somites (4, 5, 6, _ 
functional in Petromyzon); J-VIII neuromeres. 
sternum to the hyoid bone, and a geniohyoid from the hyoid to the tip 
of the lower jaw. The oblique region is also divided into three layers 
(obliques and transversus) characterized by the direction of the fibres. 
In the higher vertebrates, with the appearance of well developed ribs, 
the oblique muscles furnish the two layers of intercostal muscles, 
extending from rib to rib, and in front of the ribs they form the scalene 
muscles, extending from the ribs along the side of the neck, and the 
sternocleidomastoid, from the breast bone and clavicle to the skull. 
In the non-placental mammals a strong pyramidalis muscle extends, 
ventral to the rectus, from the inner side of the marsupial bones to the 
sternum, but disappears with these bones. 
The dorsal muscles are more conservative, undergo less modifica- 
tion than those just mentioned, and always show, more or less clearly, 
their metameric nature. They become connected with various parts 
of the vertebre and with the ribs, and are correspondingly divided into 
