188 
ANATOMY OF THE HORSE > 
Relations .—Superiorly and externally, with the preceding 
muscle; inferiorly, with the pectoralis transversus; internally, 
with the cartilages of the ribs; anteriorly, with the levator 
humeri. 
Structure .—Fleshy ; excepting its inferior termination, which 
is aponeurotic. Its fleshy parts are formed into layers, one over¬ 
lapping another. 
Action .—To assist the pectoralis magnus. 
« 
3.—COSTAL REGION. 
Comprehending muscles which have, all of them, extensive 
attachments to the ribs. 
• J ^ . * • 
{ Costo-subscapularis. 
Dorso-costalis . 
Lumbo-costalis . 
4 -v 
Situation .—Between the shoulder and the side of the chest. 
Figure .—In outline, semicircular: fibres radiating from a centre, 
and forming an indentated or serrated circumferent border. 
Attachment .—Anteriorly, to the bodies and transverse pro¬ 
cesses of the 4th, 5th, 6th and 7th cervical vertebra; posteri¬ 
orly, to the eight anterior ribs, as low down as their cartilages, 
by as many fleshy digitations: outwardly, to the upper and inner 
part of the scapula, occupying the space between the origin of 
the subscapularis and the insertion of the rhomboidei. 
Relations .—Externally, with the scapula; internally, with 
the cervical portion of the longissimus dorsi, the cervical verte¬ 
brae, and the ribs ; inferiorly, with the pectoral muscles; ante¬ 
riorly, with the splenius; and superiorly, with the latissimus 
dorsi. 
Direction .—The fibres converge from their various circum¬ 
ferent attachments, like the leaves of a fan, to one common focal 
point, which is its insertion into the scapula. 
Structure. —It is covered by a thin aponeurosis which is at¬ 
tached around, principally, to the ribs. The costal portion is 
fleshy and digitated ; the cervical portion is also fleshy ; but the 
part fixed to the scapula is mostly tendinous. The muscle is 
considerably thicker in the middle than around the circumference. 
Action .—It forms the principal agent of support to the trunk, 
maintaining it and the shoulder in close apposition. These mus¬ 
cles are more or less concerned in all the motions of the scapula; 
and will become dilators of the chest whenever they are exerted 
while the limbs remain fixed points. 
