64 ‘ ANATOMY OF THE RABBIT. 
Voluntary muscle arises chiefly from the segmented areas or 
myotomes of the embryo. The extent to which segmentation is 
shown in the adult, however, depends for the most part on how far 
the definitive muscle is removed from the vertebral 
column or segmented portions of the skeleton. 
The vertebral muscles themselves show throughout their attach- 
ments to successive vertebrae the marks of segmental origin, but 
many others, such as those of the abdomen, to a certain extent those 
of the limbs, and those of the eye show practically no indications 
of their segmental origin. 
ORIGIN. 
While the bulk of voluntary muscle 
is skeletal, part at least is related to the 
skin. This forms a eutaneous sheet, 
divisible into the 
cutaneus maximus 
and platysma, commonly developed to a 
considerable extent in mammals, but 
reduced in man, and into the facial, 
palpebral, and auricular muscles of the 
head. The trunk musculature com- 
prises a special group of cervical and 
occipital muscles in relation to the neck 
and head, and the general series which 
- are more nearly vertebral. The appen- 
Fic. 34. Arm muscles of rabbit @icular muscles are distributed in 
from the medial surface, illustrating 
muscle action, flexionandextension. special groups connecting the limb as a 
b, biceps (flexor); tr, long head of 
DISTRIBUTION. 

triceps (extensor); i. insertion: o, . whole with the trunk and the various 
origin; sc, scapula; h, humerus; 3 . 
r, radius; u, ulna. segments of the limb with one another. 
As already indicated the skeletons 
of anterior and posterior limbs are considered to be equivalent part 
for part. This is also true of the related muscles, but the respective 
EQUIVALENCE positions of the segments of the limb in a 
OF LIMB MUSCLES. ™ammal are such that in examining the 
muscles, it is important to determine the 
equivalence of the bone surfaces. Since also certain conventional 
ideas of flexion and extension prevail in human anatomy, it is 
equally important to establish the corresponding relation as between 
man and a quadrupedal form. The homologous surfaces may be 
