THE ABDOMINAL FASCL# AND MUSCLES 291 
muscle. These are termed inscriptiones tendinez. They strengthen the muscle 
and serve to prevent separation of its fibers. The width of the muscle is greatest 
about its middle. The anterior part blends with the rectus thoracis. 
Relations.—Superficially, the aponeuroses of the oblique muscles (which con- 
stitute the external rectus sheath) and the posterior deep pectoral; deeply, the 
transversus abdominis, intercostals, the cartilages of the ribs, and the sternum. 
The posterior abdominal artery runs along the lateral edge of the muscle posteriorly, 
and the anterior abdominal artery on or in its anterior part. 
Blood-supply.—Anterior and posterior abdominal arteries. 
Nerve-supply.—Intercostal and lumbar nerves. 
4. Transversus abdominis.—This muscle, named from the general direction 
of its fibers, is a triangular curved sheet. Its lateral part is muscular, its ventral 
aponeurotic. 
Origin.—(1) The medial surfaces of the distal ends or the cartilages of the 
asternal ribs, meeting the costal attachment of the diaphragm; (2) the transverse 
processes of the lumbar vertebre, by means of the deep layer of the lumbo-dorsal 
fascia. 
Insertion —The xiphoid cartilage and the linea alba. 
Action.—Similar to that of the oblique muscles. 
Structure-—The muscular part is a sheet of parallel bundles of fibers, directed 
ventro-medially. It is thickest along the cartilages of the ribs, and from here it 
thins out greatly toward the aponeurosis and the lumbar region. The fibers of the 
aponeurosis directly continue those of the fleshy part. Posteriorly it becomes ex- 
tremely thin and fades out without reaching the pelvis. It covers the deep face 
of the rectus, so forming the internal rectus sheath. 
Relations.—Superficially, the oblique and straight muscles, the retractor coste, 
the cartilages of the asternal ribs, and the internal intercostal muscles; deeply, the 
fascia transversalis and the peritoneum. The fascia transversalis is little de- 
veloped in the horse, and is very thin in emaciated subjects, but in animals in good 
condition it contains a good deal of fat. It blends with the iliac fascia and de- 
scends into the inguinal canal. The musculo-phrenic artery runs along the inter- 
val between the origin of the transversus and the costal part of the diaphragm. 
The intercostal nerves pass down over the lateral surface of the muscle, to which 
they give branches. Branches of the first three lumbar nerves are similarly dis- 
posed further back. 
Blood-supply.—tintercostal, lumbar, and musculo-phrenic arteries. 
Nerve-supply.—Intercostal and lumbar nerves. 
5. Cremaster externus.—This small muscle may be regarded as a detached 
portion of the obliquus abdominis internus, with which it blends at its origin (Figs. 
370, 575). 
Origin.—The iliac fascia, near the origin of the sartorius. 
Insertion —The tunica vaginalis communis. 
Action.—To raise the tunica vaginalis, and with it the testicle. 
Structure—The muscle arises by a thin aponeurosis which is succeeded by a 
flat muscular belly about two inches (ca. 5 em.) in width in the stallion.1 It passes 
down the inguinal canal on the postero-lateral surface of the tunica vaginalis, to 
which it is very loosely attached. On reaching the point where the tunic is reflected 
on to the tail of the epididymis, the muscle is inserted into the outer surface of the 
tunic by short tendinous fibers. 
Relations—The muscle lies between the peritoneum and the fascia trans- 
versalis in front and the iliac fascia and inguinal ligament behind. On reaching 
* As might be expected, the cremaster usually undergoes more or less atrophy and is paler 
in the castrated subject. In the mare the muscle is very small, and ends in the connective tissue 
jin the inguinal canal. 
