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To expose the abdominal muscles and the sheath of the m. rectus abdominis, the m. obliquus externus abdominis and the overlying deep fascia of the trunk on the left side are transected 2 cm ventral and parallel to the origin of the external oblique muscle from the ribs and thoracolumbar fascia (see text- illustration on p. 30). The ventral muscle remnant is reflected ventrally as far as the lateral border of the m. rectus abdominis. Then the m. obliquus internus abdominis is transected 2 cm dorsal and parallel to the border between the muscle and its aponeurosis, and also reflected.
a) Taking into consideration their sites of origin, the ABDOMINALMUSCLES
are subdivided into parts; namely, the costal part taking origin from the ribs, the sternal part from the sternum, the lumbar part from the thoracolumbar
fascia (see text-illustration p. 30), and the inguinal part from the inguinal ligament (7). With the exception of the m. rectus abdominis, each inserts at
the linea alba in the ventral midline by means of an aponeurosis or abdom- inal tendon. The m. obliquus internus abdominis also inserts along the costal arch by means of a costal tendon and the m. obliquus externus abdo- minis terminates by a pelvic tendon or lateral crus at the pecten ossis pubis. Lateral to their midventral insertions the aponeuroses of the abdominal muscles form the sheath of the m. rectus abdominis (rectus sheath, —see also p. 36), the layers passing external to the rectus abdominis forming the external lamina of the sheath, the layers passing internal to the rectus form- ing the internal lamina. Ventral and paramedian, the aponeuroses of the external lamina of the sheath form a meshlike zone that is anchored to the
tendinous intersections (2) of the m. rectus abdominis. Midventrally, the linea alba (10) arises as a consecutive series of crossing, interweaving, ten-
don fibers. It begins at the mesosternum as a ventromedian ‘anchoring raphe’ for the aponeuroses of the abdominal muscles. At its widest, it encir- cles the umbilical ring (11) by means of two umbilical crura and terminates by tapering abruptly at the cranial end of the pelvic symphysis.
I. The m. obliquus externus abdominis (8) arises as costal and lumbar parts that pass over into an abdominal and a pelvic tendon. Known respectively as the abdominal tendon or medial crus (8') and the pelvic tendon or lat-
eral crus (8''), they bound the external (superficial) inguinal ring (see p.
37). The aponeurosis of the muscle contributes entirely to the external lam- ina of the rectus sheath and inserts caudally with the abdominal and pelvic tendon at the pelvis, joining the prepubic tendon. Deep fibers of the abdominal tendon of the contralateral external abdominal oblique muscle also end on the prepubic tendon as fibrae reflexae (13).
II. The m. obliquus internus abdominis (12) has lumbar and inguinal parts. Its aponeurosis participates in the sheath of the m. rectus abdominis in three ways: 1) A cranial 2 cm-wide section of aponeurosis subscribes to the internal lamina only. 2) A subsequent 2-4 cm-wide section participates in forming both external and internal laminae in the umbilical region. 3) Caudally the aponeurosis only passes to the external surface of the m. rectus abdominis.
To demonstrate the prepubic tendon, in the accompanying figure on the right side the abdominal wall over the urinary bladder (and the prostate in males) was fenestrated. The external and internal oblique abdominal muscles, including the transversalis fascia and parietal peritoneum, are cut at the lateral bor- der of the rectus abdominis muscle and the latter is lifted caudally after being sectioned transversely at the level of the tuber coxae. In the caudal region of the left side, the abdominal tendon of the external abdominal oblique muscle is removed. The prepubic tendon is split parallel to the muscle fibers by a section between the pelvic tendon of the external abdominal oblique muscle and the medial and lateral tendons of origin of the pectineus (and long adductor) muscle. This section also cuts through the iliopubic carti lage.
III. The m. transversus abdominis (5) contributes to the sheath in a manner
similar to the m. obliquus internus abdominis. However, its contribution to both laminae occurs approximately one to two vertebral lengths more cau- dally. On the lateral surface of the muscle, ventromedial branches of tho- racic and lumbar nn. (cranial and caudal iliohypogastric and ilioinguinal nn.) run ventrally, parallel to the muscle fibers.
IV. The m. rectus abdominis (1) arises from the first rib and the first four
sternebrae and could therefore be said to have a costal and a sternal part. It ends on the pecten ossis pubis. Caudal to the sternal ribs it is enclosed in its sheath formed from the aponeuroses of the remaining three abdominal muscles and the internal and external fasciae of the trunk. The external trunk fascia contributes entirely to the external lamina and internal trunk fascia to the internal lamina of the rectus sheath except caudally where it is double-layered.
Innervation of the four abdominal muscles is by intercostal nn., by the cra- nial (3) and caudal (4) iliohypogastric nn., and by the ilioinguinal n. (6). By
means of their vm branches, all such nerves course across the lateral sur- face of the m. transversus abdominis. The vl branches of the nerves run on the external abdominal oblique muscle to the mammae (see p. 33).
8 12
5 10
Rectus sheath
(cranial abdominal region)
Deep fascia of trunk
Ext. lamina Int. lamina Transversalis fascia (transverse section) 8 12 5 10
Rectus sheaths
(middle abdominal region)
Deep fascia of trunk
Ext. lamina Int. lamina Transversalis fascia (transverse section) 8 12 5 10
Rectus sheaths
(caudal abdominal region)
Deep fascia of trunk
Ext. lamina
Int. lamina Transversalis fascia
35
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Abdominal muscles and Inguinal region
Legend :
(caudoventral view)
1 Rectus abdominis m. 2 Tendinous intersection
3 Cranial iliohypogastric n. and Cranial abdominal a. and v. 4 Caudal iliohypogastric n. 5 Transversus abdominis m.
Int. lamina Int. and ext. lamina Ext. lamina 6 Ilioinguinal nerve 7 Inguinal ligament 8 Ext. abdom. oblique m.: 8' Medial crus
8'' Lateral crus
10 Linea alba 11 Umbilical ring
12 Int. abdominal ablique m.: Int. lamina
Int. and ext. laminae Ext. lamina
Int. abdom. oblique m.: 16 Costal tendon
Sartorius m.: 17 Cranial part 17' Caudal part 18 Ext. iliac fascia 19 Iliopsoas m.
20 Adductor brevis m. 21 Adductor magnus m. 22 Gracilis m.
23 Ext. inguinal ring (supf.) 24 Int. inguinal ring (deep) 25 Transversalis fascia of trunk and
Peritoneum
a Lat. cutaneous femoral n. and deep circumflex iliac a. and v. b Lat. circumflex femoral a. and v. c Saphenous n.
d Prox. caud. femoral a. and v. e Vaginal process of peritoneum and
cremaster m. ext.
f Genitofemoral n. and ext. pudendal a. and v. g Obturator nerve h Femoral a., v. and n. i Urinarybladder j Prostate
9 Prepubic tendon
Ext. abdom. oblique m. 13 Fibrae reflexae 14 Iliopubic cartilage
15 Pectineus muscle (and long adductor)
(see p. 37)
36
a) The INGUINAL SPACE (INGUINAL CANAL) extends from the internal to the
external inguinal ring. The caudal angles of both rings lie one above the other, whereas the cranial angle of the internal ring lies approximately 2 cm craniolateral to that of the external ring. This results in a corre- sponding lengthening of the inguinal space.
I. The skin does not participate in the formation of the inguinal space and merges with the integument of the scrotum (male) or of the labia of the vul- va (female).
II. The external fascia of the trunk (5) turns at the external inguinal ring to envelop the vaginal process of the peritoneum (24) with its tubular content and is then known as the external spermatic fascia (21). The vaginal process is designated the vaginal tunic in the male (24), and con- tains the spermatic cord; whereas, in the bitch, it houses the round liga- ment of the uterus and its enveloping body of fat. In contrast to most oth- er female mammals, the bitch possesses a vaginal process of peritoneum and an accompanying external spermatic fascia in most of the cases. The
external pudendal a. and v. (20), the genitofemoral n. and the m. cre-
master (externus) pass through the space on the outside of the vaginal process and then enter the tubular external spermatic fascia. Within a few millimeters the two blood vessels and nerve pierce the fascia and the vessels branch into the caudal superficial epigastric a. and v. (see p. 14A – 9) and the ventral scrotal or ventral labial rami (see p. 33) in the vicin- ity of the superficial inguinal lymph node.
III. By a cleavage in its aponeurosis, the m. obliquus externus adominis forms the lateral crus (16) and medial crus (15) of the external (superficial)
inguinal ring (17).
The free caudal border of the m. obliquus internus abdominis (14) con- tributes to the formation of the internal (deep) inguinal ring (18) together with the lateral border of the m. rectus abdominis (19) and the internal sur- face of the inguinal ligament.
The m. cremaster (externus, —22) is distinct in the male; whereas it seems weaker in the bitch. In rodents the m. cremaster (externus, —22) is divid- ed into a primary part derived from the m. transversus abdominis and a secondary part from the m. obliquus internus abdominis. To a large degree in domestic mammals, the united m. cremaster (externus) has lost its direct connection with the two abdominal muscles. The resulting independent muscle takes origin from the inguinal ligament and passes through the inguinal space outside the vaginal process.
The m. transversus abdominis (6) does not contribute to the formation of the inguinal space since its free caudal border is at the level of the tuber coxae.
IV. The internal fascia of the trunk (7, —transversalis fascia) is adherent to the peritoneum. It protrudes through the inguinal space as the tubular
internal spermatic fascia (23) and envelops the vaginal tunic.
V. The peritoneum (8) evaginates as a tubular vaginal process, penetrates the inguinal space and is adherent to the enveloping internal spermatic fas- cia. The vaginal ring (9) in the caudolateral part of the abdominal cavity is the site of evagination and remains as the entrance into the vaginal process; it does not belong to the inguinal space.
fascia (7) extends over the m. iliopsoas (4) and is designated here as the ili- ac fascia. The inguinal ligament is woven into this fascial covering and sub-
divides the iliac fascia into an internal iliac fascia (1) on the abdominal cav- ity side and an external iliac fascia (2, —iliac lamina) on the femoral side. The two layers of fascia end on the ilium. Occasionally absent, the very weak
inguinal ligament (3) arises at the tuber coxae. In the region of the lacuna
neuromusculorum it is interwoven with the iliac fascia to provide a con- nective tissue reinforcement for the origin of the m. obliquus internus abdominis. At the level of the inguinal space more ventrally, the ligament unites with the lateral crus of the m. obliquus externus abdominis; both radiate into the prepubic tendon and end on the pecten ossis pubis. With the m. iliopsoas, the femoral n. (10) and its branch, the saphenous n. (11), pass through the lacuna neuromusculorum. This is bounded by the inguinal ligament ventrally and the ilium dorsally. Subsequently, the saphe- nous n. attains the femoral trigone (12), whose boundaries are formed by the inguinal ligament, the m. pectineus, and the caudal part of the m. sar- torius. The femoral trigone forms the borders of the femoral space (13,
femoral canal), which is bounded deeply by the m. iliopsoas and covered
superficially by the medial femoral fascia. Besides the saphenous n. the space houses the femoral a. and v.
c) The VASCULAR LACUNA (see also text-illustration) is the passage for the femoral vessels. It is bounded by the lacuna neuromusculorum dorsolater- ally, by the body of the ilium dorsomedially, and by the inguinal ligament ventrally. The medial section of the lacuna vasorum that is not occupied by blood vessels is known as the femoral ring. Due to its covering of peri- toneum and transversalis fascia, the femoral ring is a self-contained access to the femoral space (femoral canal) and is the site of femoral hernia.
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