Development of the rectus abdominis and its sheath in the human fetus

Purpose: Although the rectus abdominis and its sheath are well known structures, their development in the human fetus is poorly understood. Materials and Methods: We examined rectus abdominis and sheath development in semiserial horizontal sections of 18 fetuses at 5-9 weeks of gestation. Results: Rectus muscle differentiation was found to commence above the umbilicus at 6 weeks and extend inferiorly. Until closure of the anterior chest wall via fusion of the bilateral sternal anlagen (at 7 weeks), the anterior rectal sheath originated from the external oblique and developed towards the medial margin of the rectus abdominis at all levels, including the supracostal part. After formation of the anterior sheath, fascial laminae from the internal oblique and transversus abdominis contributed to formation of the posterior rectus sheath. However, the posterior sheath was absent along the supracostal part of the rectus abdominis, as the transversus muscle fibers reached the sternum or the midline area. Therefore, it appeared that resolution of the physiological umbilical hernia (8-9 weeks) as well as chest wall closure was not required for development of the rectus abdominis and its sheath. Conversely, in the inferior part of the two largest fetal specimens, after resolution of the hernia, the posterior sheath underwent secondary disappearance, possibly due to changes in mechanical stress. Conclusion: Upward extension of the rectus abdominis suddenly stopped at the margin of the inferiorly developing pectoralis major without facing the external intercostalis. The rectus thoracis, if present, might correspond to the pectoralis.     

Bilateral asymmetric deficiency of the pectoralis major muscle

We observed a rare, bilateral congenital deficiency of the pectoralis major muscle in a 72-year-old female cadaver in our gross anatomy dissection laboratory. The outward appearance of the anterior thoracic wall, which included well-developed breasts, revealed no obvious abnormalities. Upon dissection, the following features were observed: 1) on the left side, the sternal portion of the sternocostal head of the pectoralis major muscle was absent, the costal portion of the sternocostal head and the clavicular head were both well developed, a normal pectoralis minor was present, and the deltoid and subclavius muscles were not hypertrophied as is often the case when the pectoralis major muscle is deficient; 2) on the right side, the entire pectoralis major muscle was absent and the pectoralis minor, deltoid, and coracobrachialis muscles were infiltrated with connective tissue and fat; and 3) on both sides, the lateral pectoral nerves were absent and the medial pectoral nerves were present. The absence of the lateral pectoral nerves suggests that the deficiencies in the pectoralis major muscles are congenital malformations resulting from a developmental failure of the embryonic muscles rather than a sequel to polio or Poland's syndrome.     

The sternalis muscle: an uncommon anatomical variant among Taiwanese

The sternalis muscle is an uncommon anatomical variant. It is located on the human anterior pectoral wall, superficial to pectoralis major. This muscle has been reported both in males and females, and in whites, blacks and Asians (Barlow, 1934; Kida & Kudoh, 1991; Shen et al. 1992; Bradley et al. 1996).
Although the importance of this muscle is still a mystery, various different interpretations have been made. Clemente (1985) considered sternalis to be a misplaced pectoralis major, although some embryologists have viewed it as part of a ventral longitudinal column muscle layer arising at the ventral tip of the hypomeres (Sadler, 1995). Sadler claimed that this muscle is represented by rectus abdominis in the abdominal region and by the infrahyoid musculature in the cervical region; in the thorax, this layer usually disappears but occasionally remains as a sternalis muscle. Kitamura et al. (1985) reported a case of congenital partial deficiency of pectoralis major accompanied by an enormous sternalis. Barlow (1934), on the other hand, claimed that sternalis represents the remains of a panniculus carnosus.     

胎儿腹前壁的解剖观察

对44例妊娠3个月至足月胎儿尸体腹前壁标本,在解剖显微镜下进行了解剖观察。胎儿腹前壁的腱膜都是双层结构,每一个腹直肌鞘的前、后壁由3层腱膜组成。6层腱膜全部是斜行的,在正中线附近,两侧腱膜相互交叉编织,形成下列延续关系,即两侧腹外斜肌的2层腱膜,两侧腹横肌的2层腱膜,一侧内斜肌的腱膜与对侧外斜肌腱膜(深层)和腹横肌腱膜(前层)。此外,脐以下的部分腹横肌腱膜纤维穿过同侧腹内斜肌腱膜,与外斜肌腱膜纤维融合。脐环处还有少量纤维束附于脐索的深筋膜上。本文观察表明,腹前壁扁肌腱膜的再分层及其规则排列,在胎儿时期即已形成,这对增加腹壁的弹性和韧性,并利于腹直肌在鞘内的收缩活动具有重要功能意义。     

Innervation of the sternalis muscle accompanied by congenital partial absence of the pectoralis major muscle

In one case accompanied by congenital partial absence of the pectoralis major muscle the sternalis muscle was examined to confirm its innervation by means of analysis of intramuscular nerve distribution. It was proved that the sternalis muscle was supplied only by the pectoral nerves even in the case of sternalis in direct contact with the proper thoracic wall. These findings as well as the results of Ura (1937) and Morita (1944) favor the interpretation presented by Eisler (1901), in which the sternalis muscle was described as being supplied only by the pectoral nerves. However, the problem of double innervation of the sternalis requires continued discussion because the relationships between the pectoral nerves and the branches of the intercostal nerves or extramural nerves (Yamada & Mannen, 1985; Kodama et al., 1986) have not yet been resolved. The precise genesis of the sternalis muscle should be also examined though it has already been proved to be derived from the pectoralis muscle group including the subcutaneous trunci muscle.     

Anomalous pectoral musculature

Anomalous disposition of pectoral muscles was encountered in an adult female cadaver on the left side. A prominent cleft separating the sternocostal and clavicular portions of the pectoralis major was noticed. The fibers of pectoralis major were partially fused with the deltoid, resulting in obliteration of the deltopectoral groove. Interestingly, cephalic vein was seen traversing superficial to the clavicular portion of the pectoralis major and pierced it to drain into the axillary vein. The pectoralis minor was inserted mainly on the coracoid process and few fibers were found blending with the coracobrachialis and short head of biceps brachii. Further, pectoralis minimus, a rare anatomic variant, was also observed lying superior to pectoralis minor. It was innervated by a twig from the lateral pectoral nerve at its superficial surface. Awareness of possibility of such anomalous muscles is important for surgeons operating on the chest wall.     

Surgical anatomy of the pectoral nerves and the pectoral musculature

The pectoral nerves (PNs) may be selectively injured through various traumatic mechanisms such as direct trauma, hypertrophic muscle compression, and iatrogenic injuries (breast surgery and axillary node dissection, pectoralis major muscle transfers). The PN may be surgically recovered through nerve transfers. They may also be used as donors to the musculocutaneous, axillary, long thoracic, and spinal accessory nerves and for reinnervation of myocutaneous free flaps. Thus, in this article, we reviewed the surgical anatomy of PN. A meta-analysis of the available literature showed that the lateral pectoral nerve (LPN) arises most frequently with two branches from the anterior divisions of the upper and middle trunks (33.8%) or as a single root from the lateral cord (23.4%). The medial pectoral nerve (MPN) usually arises from the medial cord (49.3%), anterior division of the lower trunk (43.8%), or lower trunk (4.7%). The two PN are usually connected immediately distal to the thoracoacromial artery by the so-called ansa pectoralis. The MPN may also show communications with the intercostobrachial nerve. In 50%-100% of cases, it may pass, at least with some branches, through the pectoralis minor muscle. The LPN supplies the upper portions of the pectoralis major muscle; the MPN innervates the lower parts of the pectoralis major and the pectoralis minor muscle. Among the accessory muscles of the pectoral girdle, the LPN may also innervate the tensor semivaginae articulationis humero-scapularis, pectoralis minimus, sternoclavicularis, axillary arch, sternalis, and infraclavicularis muscles; the MPN may innervate the pectoralis quartus, chondrofascialis, axillary arch, chondroepitrochlearis, and sternalis muscles.     

A Taiwanese with a pair of sternalis muscles.

A pair of sternalis muscles have been found on both sides of the chest in an adult Taiwanese male. The muscles are located superficial to the medial part of the pectoralis major, arising from the sternum and are inserted into the sheaths of the rectus abdominis. They are innervated by the intercostal nerve. It should be emphasized that the sternalis muscle is rarely found in Taiwan.     

Insertion abnormality of bilateral pectoralis minimus

Bilateral insertion abnormality of pectoralis minimus (sterno-costo- coracoidian muscle) muscle was examined. The variant muscle was lying under the pectoralis major muscle and was medial to the pectoralis minor muscle. This muscle started from the first costal cartilage to the manubrium sterni and ended in the upper surface of the shoulder joint on the right side. On the opposite side, it took origin from the second costal cartilage to the manubrium sterni and the second costochondral joint, afterwards became a tendinous structure and divided into two on the coracoid process. The thicker part ended on the upper surface of the articular capsule of the shoulder joint, the thinner part inserted on the lateral third of inferior part of clavicle and fascia of subclavius muscle.     

Unilateral absence of the thoracic part of the trapezius muscle

The absence of some muscles, such as pectoralis major, serratus anterior and quadratus femoris have been described, however the absence of trapezius, either in isolation or in association with other structures, is not often observed. In this case study, we present a case of partial absence of the left trapezius muscle in an adult black male cadaver, which had a large aponeurosis replacing the ascending fibres and some of the transverse fibres of the muscle. Histological analysis showed the presence of a few muscle fibres in the aponeurosis. From a functional viewpoint, it is possible that this individual would have had difficulty in moving his shoulder, especially during flexion, abduction and circumduction of the arm.     

Complete unilateral right-sided absence of the pectoralis major and serratus anterior muscles in an adult male cadaver of South Indian origin

The unilateral complete absence of the pectoralis major and serratus anterior muscles in the absence of other anomalies is very rare. The complete absence of the pectoralis major and serratus anterior on the right side was observed during routine dissection of an elderly male cadaver of South Indian origin. Nodularity of the right fifth rib near the fifth costochondral junction was seen. The lateral pectoral and long thoracic nerves were present on the right side. Moderate right-sided disuse atrophy was noted in the pectoralis minor and the rotator cuff muscles due to a shoulder arthrodesis performed on that side. No abnormalities were noted on the left side. There was no family history of any upper limb anomalies suggestive of Poland’s syndrome. The findings in the present case could be either due to a sporadic variant of Poland’s syndrome or an isolated congenital unilateral absence of the pectoralis major and serratus anterior.     

Moment arms of the muscles crossing the anatomical shoulder

The objective of the present study was to determine the instantaneous moment arms of 18 major muscle sub-regions crossing the glenohumeral joint during coronal-plane abduction and sagittal-plane flexion. Muscle moment-arm data for sub-regions of the shoulder musculature during humeral elevation are currently not available. The tendon-excursion method was used to measure instantaneous muscle moment arms in eight entire upper-extremity cadaver specimens. Significant differences in moment arms were reported across sub-regions of the deltoid, pectoralis major, latissimus dorsi, subscapularis, infraspinatus and supraspinatus (P < 0.01). The most effective abductors were the middle and anterior deltoid, whereas the most effective adductors were the teres major, middle and inferior latissimus dorsi (lumbar vertebrae and iliac crest fibers, respectively), and middle and inferior pectoralis major (sternal and lower-costal fibers, respectively). In flexion, the superior pectoralis major (clavicular fibers), anterior and posterior supraspinatus, and anterior deltoid were the most effective flexors, whereas the teres major and posterior deltoid had the largest extensor moment arms. Division of multi-pennate shoulder muscles of broad origins into sub-regions highlighted distinct functional differences across those sub-regions. Most significantly, we found that the superior sub-region of the pectoralis major had the capacity to exert substantial torque in flexion, whereas the middle and inferior sub-regions tended to behave as a stabilizer and extensor, respectively. Knowledge of moment arm differences between muscle sub-regions may assist in identifying the functional effects of muscle sub-region tears, assist surgeons in planning tendon reconstructive surgery, and aid in the development and validation of biomechanical computer models used in implant design.     

Brief communication. Study of a supernumerary neck muscle: atlas rhomboid muscle (Macalister, 1871)

We observed a muscle originating from the lateral process of Cl and inserting on the anterior aponeurosis of the rhomboideus major muscle. This accessory muscle was present on the right side only.     

Sternalis muscle,what every anatomist and clinician should know

The sternalis muscle is a well documented but rare muscular variation of the anterior thoracic wall. It lies between the superficial fascia and the pectoral fascia and is found in about 8% of the population. It presents in several morphological variants both unilaterally and bilaterally and has no apparent physiological function. There is still much disagreement about its nerve supply and embryological origin. With the advent of medical imaging and thoracic surgery the clinical importance of this muscle has been re‐emphasized. It has been implicated in misdiagnosis of breast masses on routine mammograms owing to its parasternal location and relative unfamiliarity among radiologists. When undetected before any thoracic surgery, it has the potential to interfere with and prolong such procedures. When present and detected preoperatively it can be used as a muscular flap in reconstructive surgeries of the breast and neck. This article will present the sternalis muscle with special emphasis on its morphology, homology, and clinical significance. Clin. Anat. 27:866–884, 2014. © 2014 Wiley Periodicals, Inc.     

Case reports on the pectoralis quartus and the pectoralis intermedius muscles

Each one case of the pectoralis quartus and the pectoralis intermedius muscles was found on the left thoracic wall in a 73 year old male and on the right thoracic wall in a 57 year old female respectively. The pectoralis quartus was a thin muscle of triangular shape, the base of which was the origin arising from the left thoracic wall at the level of the 6th rib between the pectoralis major and the latissimus dorsi, being separated from both margins of the muscles. The aberrant muscle ascended left-upwards about 10 cm to insert to the inner surface of the pectoralis major near its lower margin. The muscle was innervated by the most caudal pectoral nerve, passing around the lower margin of the pectoralis minor. The pectoralis quartus is extremely rare in man, and only two cases were reported by Bluntschli (1906) and Frey (1921). From the comparative anatomical point of view, the pectoralis quartus muscle was supposed to be a remnant of the ventral part of the subcutaneous trunci muscle in lower mammals, differing from the ordinary muscular arch of the axilla which was believed to derive from the dorso-cranial part of the muscle. The pectoralis intermedius was located in the deep layer of the right pectoralis major, lying about 2 cm below the lower margin of the pectoralis minor. It arose in the 4th and 5th ribs and extended right-upwards, running almost parallel with the pectoralis minor. The origin of the pectoralis minor shifted cranially to the 2nd and 3rd ribs.(ABSTRACT TRUNCATED AT 250 WORDS)     

The muscular arch of the axilla and its nerve supply in Japanese adults]

We examined 94 axillary regions of 47 Japanese adults and found the muscular arch of the axilla (Maa) in five sides of three cadavers as well as the tendinous arch of the axilla (Taa) in two sides of two cadavers. The results are summarized as follows: 1) The frequency of Maa was 6.4% of the total bodies and 5.3% of the sides in this series. 2) In the left side of a 57-year-old male (No. 427), Maa was attached to the surface of the coracobrachialis muscle after fusing with the dorsal surface of the inserting tendon of the pectoralis quartus muscle. Both muscles were supplied by the caudal pectoral nerve (Npc) from the medial pectoral nerve. Moreover, in this same specimen, the sternalis muscle was recognized on the ventral surface of the pectoralis major muscle. In the left side of a 93-year-old female (No. 386), the cranial part of the muscular arch of the axilla (Cpa) was extended to the coracoid process by a tendon and attached to the abdominal part of the pectoralis major by two muscle bundles supplied by independent branches from Npc. One muscle bundle was attached to the lower margin of the abdominal part of the pectoralis major on the same plane, and the other bundle was located on the dorsal surface of the abdominal part. In a 74-year-old female (No. 411), the well-developed lateral part of the muscular arch of the axilla (Lpa) was attached to the inferior side of the tendinous arch. According to Ruge (1914) and Kasai et al. (1977), this arch was in the transition of the muscle bundle of Cpa to the arch. In the right side of the same specimen, only the thoracodorsal nerve (Ntd) was distributed into Lpa, whereas in the left side, only Npc supplied branches to Lpa. 3) The axillary arch was classified into 8 types based on the form and the supplying nerve of Cpa and Lpa. Cpa consisting of the muscle bundle is Type I, and Cpa consisting of the tendinous arch is Type II. We proposed that only Type II-A, with Cpa as tendinous arch and no Lpa, be designated as Taa (found in two cases), and the others as Maa. The following types were found in this study: Type I-A, consisting of only Cpa supplied by Npc (two cases); Type I-D, consisting of Cpa supplied by Npc and Lpa supplied by Ntd (one case); Type II-B, consisting of the tendinous arch and Lpa supplied by Npc (one case); Type II-D, consisting of the tendinous arch and Lpa supplied by Ntd (one case). 4) From the above findings, it can be suggested that Maa of varying shapes have been formed by a portion of the latissimus dorsi muscle supplied by Ntd, together with the pectoralis subcutaneous muscle, consisting of the pectoralis abdominalis, humeroabdominalis, humerodorsalis and ventrolateralis muscles supplied by Npc. The latter three muscles were proposed by Ura (1937) as the panniculus carnosus muscle, which was well developed in some lower mammalian orders. However, early investigators suggested that Maa was derived from the panniculus. Maa might have occurred as a rudimentary phylogenetic remainder in an early human embryonic stage.     

Sternalis muscle: importance in surgery of the breast

Mastectomy is a commonly performed surgery for carcinoma of breast. During surgery, pectoral fascia is removed and pectoralis major muscle is laid bare. Sternalis is a rare muscle encountered in the subcutaneous plane. We examined the operative records of 1,152 patients who underwent modified radical mastectomies between 1990 and 2000. Patients who underwent conservative breast surgery or radical mastectomy were excluded. Among 1,152 patients who underwent modified radical mastectomy, eight were identified as having sternalis, a subcutaneously placed muscle oriented craniocaudally. The thickness of the muscle varied. The muscle was spared in all patients. Sternalis is a rare muscle in the subcutaneous plane. It should not be mistaken for a mass on mammography. During surgery it is important to be aware of this rare entity and identify the muscle early so that the dissection plane is appropriate. The depth at which internal mammary nodes are irradiated may also vary in the presence of the muscle. In addition, it should not be mistaken for recurrence on follow-up.

---

Electronic Supplementary Material The french version of this article is available in the form of electronic supplementary material and can be obtained by using the Springer Link server located at .

---

Le muscle sternal: importance dans la chirurgie du sein

Résumé La mastectomie est le procédé chirurgical le plus souvent réalisé pour traiter le cancer du sein, comportant l'ablation du fascia du muscle grand pectoral qui est mis à nu. Le muscle sternal est rarement retrouvé dans le plan sous-cutané. Sur un total de 1,152 mastectomies radicales modifiées (1990–2000), excluant les gestes de conservation mammaire et les mastectomies radicales classiques, un muscle sternal sous-cutané, à orientation crânio-caudale et d'épaisseur variable, a été trouvé dans 8 cas et toujours préservé. Le muscle sternal peut être confondu avec une tumeur sur la mammographie. Il est important de le repérer rapidement dans le plan sous-cutané pour utiliser le bon plan de clivage. Il peut modifier la technique d'irradiation des noeuds lymphatiques parasternaux et être considéré à tort comme une récidive tumorale.

---

     

Coexistence of a pectoralis quartus muscle and an unusual axillary arch: case report and review

A pectoralis quartus muscle and an unusual axillary arch were found on the left side of a female cadaver. The axillary arch was a musculoaponeurotic complex continuous with the iliacal fibers of the latissimus dorsi. The muscular part, together with the tendon of pectoralis major, inserted into the lateral lip of the bicipital groove of the humerus, whereas the aponeurotic part was formed by a fibrous band that extended deep to the pectoralis major to insert into the coracoid process between the attachments of the coracobrachialis and pectoralis minor. The pectoralis quartus originated from the rectus sheath, and joined the inferior medial border of the fibrous band of the axillary arch, at the lateral edge of the pectoralis major. The axillary arch muscle crossed anteriorly the axillary vessels and the brachial plexus. The clinical importance of these muscles is reviewed.     

骨盆骨折髂腹股沟入路避免神经血管损伤的临床解剖学

目的　为髂腹股沟入路在骨盆骨折手术中避免神经血管损伤提供解剖学依据。  方法    在15具(男9具,女6具)30侧成尸标本上选择髂前上棘、耻骨结节和腹股沟韧带为标志观测股外侧皮神经(Lateral femoral cutaneous nerve,LFCN)、髂腹股沟神经(Ilioinguinal nerve,IN)、股神经耻骨肌支和闭孔动脉(Obturator artery,OA)的走形特点,所测数据统计学处理。  结果     ① LFCN在髂前上棘内侧穿出腹股沟韧带占96.67% (29/30侧),距髂前上棘中心点(20.01±0.32)mm;被腹股沟韧带纵横纤维所包裹的占33.33% (10/30侧);在阔筋膜形成的筋膜鞘中走行占46.67%(14/30侧)。② IN穿出腹内斜肌部位距离髂前上棘中心点为(5.41±0.50)mm,穿出腹外斜肌腱膜部位距离耻骨结节中心点为(18.04±0.21)mm。  结论　在显露髂骨翼内侧面和骶髂关节时,应在LFCN走行的阔筋膜和腹股沟韧带部位进行显露和预防性松解,以免牵拉损伤;切开腹外斜肌腱膜时应从腹股沟韧带两端上方5 mm处开始,防止损伤深面的IN。在显露髂耻隆起时先寻找和结扎闭孔血管耻骨支,以免引起不可控制的出血。