Talk:Paper - The sternalis muscle in the anencephalous foetus (1936)
ANATOMICAL NOTES THE STERNALIS MUSCLE IN THE ANENCEPHALOUS FOETUS
By W. F. HARPER From the Department of Anatomy, University of Aberdeen
Srx anencephalous foetuses were dissected and the sternalis muscle was found in three,
(1) Full-time female foetus, anencephalous with spina bifida in lower dorsal and lumbar regions. Sternalis unilateral, right. Fig. 1.
The muscle arose by fleshy fibres from the fascia over the lower part of the manubrium and upper part of the body of the sternum and from the third right costal cartilage. In this situation its fibres were firmly blended with those of the pectoralis major. The muscle passed downwards and outwards to be inserted mainly into the aponeurosis of the external oblique. The distal part of the muscle had no direct connection with the pectoralis major which lay deeply to it, but a few of its fibres passed on to the deep fascia over the attachment of that muscle to the sixth costal cartilage. The length was 3-5 cm. and the maximum breadth 1 cm. The upper
part of the pectoralis major of this side was well developed, but its lower portion was greatly thinned particularly towards the sterno-costal attachment. A triangular gap was present between these two portions. This gap was filled with loose areolar tissue and partly covered by the intermediate portion of the sternalis.
The left pectoralis major and the platysma were well developed.
The nerve supply was derived from the medial anterior thoracic. This nerve pierced the pectoralis minor, crossed the gap in the pectoralis major and entered the outer border of the sternalis. Terminal twigs of the third intercostal nerve pierced the muscle on their way to the skin.
(2) Full-time female foetus, anencephalous with spina bifida in lumbar region. Sternalis bilateral. Fig. 2.
The condition here was almost bilaterally symmetrical. The origin of each muscle was from the upper part of the body of the sternum and from the second and third costal cartilages. In this situation there was firm union with the fibres of the pectoralis major. The direction of the fibres in both was downwards and outwards to an insertion 318 Anatomical Notes
on the fifth costal cartilage and the adjacent part of the fifth rib. A well-marked triangular gap was present in the intermediate part of each pectoralis major, but in contrast to case 1 the fibres of insertion of each sternalis lay deep to those of the lower portion of that muscle. The length was approximately 8 cm. and the maximum breadth 1-2 cm. in each case. There was no connection with the platysma or sternomastoid.
The nerve supply was from the medial anterior thoracic in both and reached the muscles after a course similar to that in case 1.
(3) Full-time female foetus, anencephalous with spina bifida in cervical and thoracic regions. Sternalis unilateral, right. Fig. 3.
The sternalis in this case was not of large size. It arose by fleshy fibres from the fascia over the upper part of the manubrium and from the right sterno-clavicular joint. In this region it was firmly fused to the pectoralis major. From this attachment the muscle passed downwards almost vertically, narrowing as it descended to an insertion on the fourth costal cartilage. The pectoralis major of this side exhibited a large triangular deficiency which was only partly covered by the lower fibres of the sternalis. In the gap the fibres of origin of the pectoralis minor could be seen. Anatomical Notes 319
The clavicular origin of the pectoralis major of the same side was limited to a small area at its medial end, while its lower portion arose from the sixth and seventh costal cartilages. Near the insertion of the muscle into the humerus the two portions joined almost at right angles. The deltoid was normal and well developed, but owing to the small clavicular attachment of the pectoralis major an appreciable interval, in which the tip of the coracoid process could be seen, was present between the adjacent borders of these muscles.
Both the lateral and medial anterior thoracic nerves were observed entering the pectoralis major but no distinct twig was traced to this abnormal muscular slip.
The term ‘‘sternalis muscle” has been employed in these descriptions, as the examples resemble those similarly named by Shepherd (8, 9) and others. The appearance of the parts and the innervation of the muscles, however, clearly indicate that they have been formed by a deviation or dislocation of some of the fibres of the pectoralis major.
The sternalis has been variously described as a vestige of the panniculus adiposus, a homologue of the sterno-cleido-mastoid, a portion of the pectoralis major, and as an extension upwards of the rectus abdominis. These views have been discussed by Turner (11), who considered that the evidence was in favour of classifying it with the platysma. Some evidence in favour of regarding certain cases as derivatives of the pectoralis major has been adduced, particularly by Bardeleben, Shepherd, Cunningham (3,4), Windle, and Abraham. Bardeleben (2) was the first to enunciate this view, but as he observed twelve cases in adults all innervated by intercostals he restricted those derived from this source to 21 per cent. Shepherd (8, 9) traced the nerve supply to ten sternalis muscles occurring in anencephalous foetuses; all were supplied by the anterior thoracics, but two also received an additional supply from intercostals. In every case where the pectoralis major was deficient the nerve supply was derived from the anterior thoracics alone. In the two cases which received an additional supply from the intercostals there was no such deficiency. This led him to consider that there were probably two different kinds of sternalis.
As indicating the relationship of the muscle to the pectoralis major, Windle (12) and Abraham (1) also stressed the fact that in the anencephalous foetus in particular, the presence of the muscle was frequently associated with a thinning or an actual gap in the pectoralis major.
Hallet (6), in 1848, seems to have been the first to record the nerve supply to the sternalis in an adult; it was from the third, fourth, and fifth intercostals. Dwight (5) recorded five cases occurring in adults. Of these, two were innervated by intercostals, one by one of the ? anterior thoracic nerves, while in two it could not be made out. The same author also described a bilateral sternalis associated with a definite interspace in both large pectorals but did not give its nerve supply.
Cunningham (4), in 1888, tabulated the source of the nerve supply in cases recorded up to that date, and found that in thirty-three it was derived from the anterior thoracics and in five from the intercostals.
Smith (10) recorded a bilateral sternalis in an adult supplied by the second and third intercostals and was certain that no twigs entered it from the thoracics. Recently, I observed a bilateral sternalis in an adult dissecting room subject which agreed in size, position and innervation with that described by Smith(10). It was entirely superficial to well-developed pectoral muscles and deep to the platysma.
. Patten (7) recently described two cases of right sternalis in adult dissecting room subjects both innervated by the external anterior thoracic nerve.
No satisfactory explanation can be advanced as to the remarkable frequency with which this abnormality occurs in the anencephalous foetus. The fact that its nerve supply may be derived from one or other of the anterior thoracics shows that the same set of fibres is not always implicated in its formation. Further, the varia320 Anatomical Notes
bility of its nerve supply, particularly in the adult, appears to indicate that the muscle is not constantly derived from the same source.
Three examples of sternalis muscle in the anencephalous foetus are described. The appearance of the parts and the innervation of the muscles indicate that they have been derived from the pectoralis major.
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