THE ABDOMINAL FASCIA AND MUSCLES 291 



muscle. These are termed inscriptiones tendineae. 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 vertebrae, 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 fieshy 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 costse, 

 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. — Intercostal, lumbar, and musculo-phrenic arteries. 



Nerve-supply. — Intercostal and lumbar nerves. 



5. Cremaster extemus. — 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 cm.) in width in the stallion.^ 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 



1 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 

 in the inguinal canal. 



