A Case of the Scalenus Anterior Muscle. Passing Behind the Left Subclavian Artery

Transcription

1 Okajimas Folia Anat. Jpn., 66 (5): , December, 1989 A Case of the Scalenus Anterior Muscle Passing Behind the Left Subclavian Artery By Norihisa INUZUKA Department of Anatomy, University of Tokyo, Faculty of Medicine, 7-3-1, Hongo, Bunkyo-ku, Tokyo, 113 Japan - Received for Publication, April 25, 1989 Key word: Anomaly, Scalenus anterior muscle, Subclavian artery, Brachial plexus Summary: The scalenus anterior muscle was found to pass behind the left subclavian artery and the first thoracic nerve in a 95-year-old Japanese woman. The scalenus anterior muscle originates from the fifth and sixth cervical vertebrae and inserts on the first rib more dorsal than typical. It is innervated by the fifth and seventh cervical nerves. The muscle belly is thin. The scalenus minimus was not found. The left vertebral artery originates from the aortic arch and enters the transverse foramen of the fifth cervical vertebra. The primary vertebral artery arises from the costocervical artery. The internal thoracic artery originates from the subclavian artery more distally than typical. The axillary artery crosses the brachial plexus between the eighth cervical and first thoracic nerves. Because the first thoracic nerve joins the brachial plexus more distally than usual, the plexus has no typical inferior trunk. Comparative anatomy shows that the muscles, nerves and arteries of the lateral cervical region of the present case maintains primitive characteristics. From the functional viewpoint, the mechanical efficiency of the scalenus anterior muscle is probably lower than usual due to the lower point of origin and the dorsal shift of the insertion. The scalenus anterior originates from the anterior tubercles of the third to sixth cervical vertebrae and inserts at the scalene tubercle of the first rib. The subclavian artery and the root of the brachial plexus pass through the space between the scalenus anterior and the scalenus medius. However, during dissection practice, we found an example in which the left scalenus anterior was located behind the subclavian artery and the first thoracic nerve. The cadaver was a 95-year-old Japanese female, and the cause of death was decrepitude. In the present case, the left scalenus anterior was poorly developed and terminated at the scalene tubercle which was situated more dorsal than typical. The scalenus minimus was not found. The usual brachial plexus and inferior trunk was not found. The first thoracic nerve does not join the posterior fascicle. The left vertebral artery arose from the aortic arch. The axillary artery crossed the brachial plexus between the eighth cervical and the first thoracic nerve (Fig. 1, 2). Such a case has not been reported to date. This anomaly was found only on the left side; there were no other anomalies in other parts of the body. 229

2 230 N. Inuzuka Findings 1) The left scalenus anterior muscle The muscle originated from the fifth and the sixth cervical vertebrae. A thin muscle arose from the dorsomedial margin of the scalenus medius and joined the scalenus anterior at its dorsolateral side. The muscular head from the fifth cervical vertebra was thin and originated from the lower end of the anterior tubercle as a tendinous fiber. The bulk of the muscle was mainly composed of the head from the sixth cervical vertebra, which originated as a muscular and tendinous fiber from the lower end of the anterior tubercle and from the inferolateral end of the groove for spinal nerve. The maximum width of the muscle belly was 11 mm and the thickness 4 mm. The width of the tendon of the scalene muscle tail was 7.5 mm. The muscle was inserted at the scalene tubercle of the first rib. The direct distance between the medial margin of the tubercle and the costal head was 34 mm. The muscle passed in front of the sixth to the eighth cervical nerves and behind the Fig. 1. Anterior view of the anomalous left cervical region. The subclavian artery and the first thoracic nerve pass in front of the scalenus anterior muscle. The axillary artery passes between the eighth cervical and the first thoracic nerves. The internal thoracic artery surrounds the phrenic nerve.

3 Scalenus Anterior Behind Subclavian Artery 231 anterior branch of the first thoracic nerve and the subclavian vessels. The muscle was innervated by the fifth and seventh cervical nerves (C5, C7). 2) The right scalenus anterior muscle The muscle originated from the lower and the lateral ends of the anterior tubercle of the sixth cervical vertebra as a muscular and tendinous fiber. The maximum width of the muscle belly was 14 mm and the thickness 7.5 mm. The tendon of the muscle tail was 6 mm. The muscle was inserted at the scalene tubercle of the first rib. The direct distance between the medial margin of the tubercle and the first rib head was 41 mm. The muscle followed the typical course, passing in front of the sixth cervical to the first thoracic nerves and the subclavian artery, and behind the subclavian vein. The muscle was innervated by the sixth to the eighth cervical nerves (C6-C8). 3) The left scalenus medius muscle The muscle originated from the atlas to the sixth cervical vertebrae. The muscle head from the lower surface of the basal part of the transverse process of the atlas was muscular, tendinous and thin. The most distinct head was tendinous and originated from the lower end of the groove for spinal nerve of the transverse process of the fourth cervical vertebra. The other heads were thin and very Fig. 2. Anterior view of the normal right cervical region.

4 232 N. Inuzuka short. The maximum width of the muscle belly was 20 mm, and the thickness 9 mm. The width of the tendon of the muscle tail was 20 mm. The muscle was inserted into the proximal part of the first rib. The direct distance between the lateralmost margin of the muscle insertion and the costal head was 40 mm. The muscle attachment on the rib in front of the levatores costarum muscles was thin and tendinous. The muscle ran longitudinally behind all the cervical nerves. One of the branches from the fifth cervical nerve penetrated the muscle. The muscle was innervated by the fifth to seventh cervical nerves (C5-C7). 4) The right scalenus medius muscle The muscle originated from the atlas to the seventh cervical vertebrae. Heads of the right muscle were larger than those of the left. The muscle was not generally divided into independent muscle heads except for the lower part. The muscle head from the transverse process of the atlas was thin and tendinous. Main origins of the muscle were those from the lower ends of the groove for spinal nerve of the transverse processes of the fourth and fifth cervical vertebrae. The maximum width of the muscle belly was 21 mm, and the thickness 9 mm, and the width of the muscle tail tendon was 17 mm. The muscle was inserted into the proximal part of the first rib, at 46 mm from the costal head, as measured directly. The insertion was tendinous and surrounded the anterolateral side of the levator costae. It can longitudinally behind all the cervical nerves and the first thoracic nerve, and was penetrated by some branches from the fifth cervical nerve. The muscle was innervated the fourth and fifth cervical nerves (C4, C5). 5) The left brachial plexus The plexus consisted of five roots of ventral rami from the fifth cervical (C5) to the first thoracic nerve (Th1). Among these C5 and C6 formed the superior trunk and the other roots ran independently. The fifth to the eighth cervical nerves (C5-C8) originated from the scalene space, however, the first thoracic nerve ran in front of the scalenus anterior along with the subclavian artery and vein. The superior trunk divided into three: the suprascapular nerve first, and then the anterior and posterior divisions. The anterior division joined the anterior divisions of C7 and C8. Likewise, the posterior divisions joined. The anterior division of C7 joined that of C8. All the anterior divisions joined to forn a bundle, which we call "the anterior bundle". "The anterior bundle" branched out, sending off the pectoral nerves, and then bifurcated to form the medial and the lateral branches. The medial branch joined the first thoracic nerve (Thl) to become the medial cord, and the lateral branch became the lateral cord. The three posterior divisions formed the posterior cord. As Thl did not join C8, it was not included in the posterior cord. The medial cord sent off the medial brachial and antebrachial cutaneous nerves just after the cord was formed, and then divided into the ulnar nerve and the medial root of the median nerve. The lateral cord sent off the musculocutaneus nerve and then became the lateral root of the median nerve. The posterior cord became the radial nerve after issuing the axillary nerve. The relationship of the plexus and the axillary artery was peculiar in that the artery ran between the eighth cervical and the first thoracic nerves, 72 mm from the median line. 6) The right brachial plexus The plexus consisted of five roots of ventral rami from C5 to Thl, which originated together with the subclavian artery from the scalene space. The composition of the plexus was typical.

5 Scalenus Anterior Behind Subclavian Artery 233 The medial cord sent off the medial brachial and antebrachial cutaneus nerves and the ulnar nerve at the same level, and received a fine branch from the lateral bundle. The medial cord then ran 110 mm before joining the lateral root of the median nerve. The lateral cord did not immediately send off the musculocutaneus nerve, rather sent off four fine branches. Then it became the lateral root of the median nerve and joined the medial root to form the median nerve. The point of formation of the median nerve was located at the distal third of the upper arm. The posterior cord sent off the axillary nerve and as the radial nerve issued fine branches toward the ulnar nerve. These fine branches, however, ran along the ulnar nerve and soon separated to enter the triceps brachii muscle (N. collateralis ulnaris n. radialis). The axillary artery entered between the medial and lateral cords at 94 mm from the median line. 7) The left subclavian artery The artery originated from the aortic arch as the fourth branch. The diameter at the origin was 8.5 mm. The left vertebral artery arose from the aortic arch as the third branch, originating (3.1 mm diameter) slightly dorsal to the common carotid artery and just ventral to the subclavian artery. The artery ascended straight behind the common carotid artery for 88 mm and then entered (2.5 mm diameter) into the transverse foramen of the fifth cervical vertebra. The subclavian artery ascended from the base and began to bend outward at approximately 25 mm from the origin, and then changed direction slightly downward at 37 mm from the origin. The subclavian artery crossed the first rib just after the origin of the internal thoracic artery. The first branch of the subclavian artery, the costocervical trunk, diverged upward and had a diameter of 2 mm at 40 mm from the base. It ascended for 10.5 mm and trifurcated (Fig. 3). The medial branch ran upward in front of the brachial plexus and immediately bifurcated. One of the bifurcations entered into the transverse foramen of the sixth cervical vertebra with the vertebral vein (the primary vertebral artery). The other bifurcation ascended and passed below the seventh cervical nerve, and then became an artery corresponding to a branch of the deep cervical artery which ran dorsally between the transverse processes of the sixth and seventh cervical vertebrae. The intermediate branch (the deep cervical artery) ran backward immediately after division and passed between the eighth cervical nerve and the first rib and then entered into the dorsal muscles passing below the transverse process of the seventh cervical vertebra. The lateral branch (the supreme intercostalis artery) descended and ran below the first rib and the first thoracic nerve. The second branch of the subclavian artery, the thyroid axis, diverged upward and had a diameter of 3.2 mm at 44 mm from the base. First, the inferior thyroid artery diverged inward from the thyroid axis at 12 mm from the divergence. Then, the axis bifurcated into the ascending cervical artery and the transverse cervical artery, which had a diameter of 2 mm at 3 mm from the origin of the inferior thyroid artery. The third branch of the subclavian artery, the internal thoracic artery, diverged caudally and had a diameter of 2.5 mm at 7.3 mm from the second branch. Because the internal thoracic artery was more distally situated than in typical cases, the phrenic nerve, descending obliquely outward and winding around the outer side of the internal thoracic artery, entered into the thoracic cavity. 8) The right subclavian artery The brachiocephalic artery originated (12 mm diameter) from the aortic arch as the first

6 234 N. Inuzuka branch. The right subclavian artery separated from the common carotid and had a diameter of 10 mm at 55 mm from the base. The subclavian artery ran nearly horizontally backward and outward, it turned downward at approximately 25 mm from the base, and then crossed the first rib at 35 mm. The first branch of the subclavian, the vertebral artery, diverged from the superior posterior surface and has a diameter of 4.5 mm at 3 mm from the origin of the subclavian artery. The vertebral artery ascended behind the common carotid artery with a meandering course, and then entered the transverse foramen of the fifth cervical vertebra. The diameter at 46 mm from the divergence was 3.5 mm. The second branch of the subclavian artery, thethyroid axis, diverged upward and had a diameter of 5 mm at 13 mm from the first branch. After ascending 5 mm, the thyroid axis separated into three arteries, the inferior thyroid, the ascending cervical and the transverse cervical. The internal thoracic artery diverged caudally (3 mm diameter) at nearly the same point as the second branch. The costocervical artery diverged backward and upward and had a diameter of 1 mm at 10 mm from the thyroid axis and the internal artery. Discussion I. Frequency of the anomaly Adachi (1928) reported, in 2 out of 328 Japanese cases (0.6 ± 0.43%) and 4 out of 340 European cases (1.2 ± o) the subclavian artery passed in front of the scalenus anterior. Two additional cases have been documented in Japan (Adachi, 1928; Fig. 151), and in one of these cases the first thoracic nerve also passed in front of the scalenus anterior. The latter case belongs to the so-called Adachi's C-type in which the axillary artery does not pass through the brachial plexus. In a case described by Dolgo-Saburoff (1933), only the subclavian artery passed in front of the scalenus anterior and the brachial plexus ran behind. Forster (1916), Adachi (1928), Dolgo- Saburoff (1933) and Kanda (1978) suggested that these anomalies are due to a transfer of the muscle insertion. In the present case, the scalenus anterior originated from C5 and C6. The frequency of this case is 19% (Serizawa, 1968), thus not so infrequent. However, the axillary artery crosses the brachial plexus between C8 and Thl. This anomaly has been reported by Miller (1939) in only 4 out of 480 cases (0.8307o) and by Yamada (1975) in one out of 50 cases (2%). Although the brachial plexus is typically composed of five roots, it is rare that the first thoracic nerve does not join the posterior cord. This type of anomaly has been reported by Hirasawa (1931) in one out of 200 cases (type VI II 3) and by Arakawa (1952) in one out of 450 cases (type X2 B, 0.22%). As mentioned above, the present anomaly is very rare. The morphology of this anomaly will be considered from the embryological, topographical, comparative and functional anatomical viewpoints. II. Embryology A. The anomaly of blood vessels Miller (1905) explained the relationship between the axillary artery arid the brachial plexus from the viewpoint of embryology and comparative anatomy. He hypothesized various perforating relationships owing to the disappearance of some parts or branches in the arterial axillary plexus perforate the brachial plexus, namely, either the medial longitudinal trunk (mediale Langsstamm), the lateral longitudinal trunk (lateral Langsstamm) or the sixth to ninth intersegmental

7 Scalenus Anterior Behind Subclavian Artery 235 arteries that connect these trunks like the steps of a ladder. According to this theory, the typical axillary artery perforating between C7 and C8 is derived from the seventh intersegmental artery and the present axillary artery, between C8 and Thl, is derived from the eighth intersegmental artery. The subclavian artery in the present case is peculiar in that the vertebral artery arises directly from the aortic arch, the costocervical artery gives off the primary vertebral artery (which is very thin in this case), and the internal thoracic artery diverges more distally than in typical cases. These features are reasonably explainable on the assumption that the vertebral artery is derived from the sixth intersegmental artery and the subclavian artery from the eighth. The costocervical artery (Fig. 3, 4) ascends and then forks into three branches. One branch, after further ascending, bifurcates into the primary vertebral artery, entering the transverse foramen of the sixth cervical vertebra (indicating that the artery is derived from the seventh intersegmental artery) and the artery corresponding to the deep cervical Fig. 3. The left costocervical trunk and its branches.

8 236 N. Inuzuka artery passing between the transverse processes of the sixth and the seventh cervical vertebrae (thus derived from the seventh intersegmental artery). The second branch becomes the deep cervical artery passing under the eighth cervical nerve and the transverse process of the seventh cervical vertebra (thus derived from the eighth intersegmental artery). The third branch becomes the supreme intercostalis artery which is distributed under the first rib and the first thoracic nerve (thus derived from the ninth intersegmental artery). If the subclavian artery in this case was typically derived from the seventh intersegmental artery, three branches must diverge at the same time: the primary vertebral artery, the branch corresponding to the deep cervical artery at the level of the seventh intersegmental artery, and the branch corresponding to the essential costocervical artery derived from the eighth intersegmental artery. Therefore, it can be concluded that the subclavian artery in question is derived from the eighth intersegmental artery. The internal thoracic artery branches so distally that it goes around the phrenic nerve. Typically the anterior branch of the eighth and the following intersegmental arteries run along the anterior thoracic wall. Therefore, the position of the internal thoracic artery also supports the conclusion mentioned above. Fig. 4. Embryological schema of the left subclavian and the vertebral arteries, the costocervical trunk and the thyroid axis. Mode of origin of branches from the costocervical trunk suggests that the subclavian artery is derived from the eighth intersegmental artery. B. Embryology of muscles Eisler (1912) has classified the scalenus muscles into three types by origin and insertion. The scalenus anterior is defined as that which originates from the ventral tuberculum of transverse process from the third to sixth cervical vertebrae and inserts on the scalene tuberculum of the first rib. The muscle bundle separating the inferior trunk of the brachial plexus is- thus regarded as the scalenus anterior, although it has relatively few muscle heads and a thin belly. The muscle definitely inserts on the scalene tubercle, but the tubercle itself develops on the point of muscle insertion, so the definition is tautological. The insertion in question is situated more dorsally on the left side than on the right side, and the scalene space between the scalenus anterior and the typical scalenus

9 Scalenus Anterior Behind Subclavian Artery 237 medius is so narrow that both muscles attach to each other at their insertion (Fig. 1). Therefore, the left scalenus anterior in question is probably due to dorsal transposition of the muscle insertion. From the above-mentioned facts, it is clear that there are anomalies in the first thoracic nerve, the subclavian artery and the scalenus anterior in the present case. According to Keibel and Mall (1910; 1911), nerves are not observed yet in the upper limb buds of the 4.5 mm fetus, the subclavian artery is defined already in the 5 mm fetus, and each of the scalenus muscles in the 11 mm fetus lies in the same topographical relationships as in adult. However, data of earlier stages is unavailable. Consequently the primary factor causing these anomalies cannot be definitely defined. III. Topographic anatomy Even if the routes of vessels and nerves are typical, in cases in which the scalenus anterior inserts further back than in the typical, anomalies can appear in the topographical relationships. Considering the relative position of muscles, including the scalenus anterior and the scalenus minimus to the subclavian artery and the brachial plexus, these anomalies have been summarized into various series including examples shown by Serizawa (1968) and Unehira and Murakami (1970) as shown in Figure 5. The present case is a very extreme example among these series. The explanation as to why only the first thoracic nerve runs in front of the scalenus anterior is as follows. The route of the scalenus anterior is from the fifth and sixth cervical vertebrae outward and downward. The upper nerve roots composing the brachial plexus cross the muscle proximally. This situation makes the topographical relationship of the lower roots more varied. The origin of the scalenus anterior is the anterior tubercle of the transverse process of the fifth and sixth cervical vertebrae. The roots of the brachial plexus pass through the grooves for the spinal nerve. The anterior Fig. 5. A series of various of the insertions of the scalenus anterior. The present case is furthermost right side.

10 238 N. Inuzuka tubercle of the transverse process is anterior to the grooves for spinal nerve. So the origin of the scalenus anterior is always anterior to the brachial plexus. On the other hand, the position of the scalene tubercle on the upper surface of the first rib can vary greatly. For this reason, in when the scalenus anterior attaches near the scalenus medius muscle, it becomes more difficult for the inferior nerve to pass due to the narrow inverse triangularshaped scalene space. Furthermore, it seems that the position of the first thoracic nerve can easily vary because it runs upward after passing along the medial margin of the first rib. IV. Comparative anatomy Forster (1916) examined the scalenus from the viewpoint of comparative anatomy. He maintained that the scalenus medius exists even in the lowest mammals, but the scalenus anterior appears first in marsupials. The muscle passes behind the brachial plexus in prosimians and in front in platyrhini. The origin of the scalenus anterior also reaches the upper cervical vertebrae in higher primates. The scalenus anterior from the fifth and the sixth cervical vertebrae, as in the present case, is observable in platyrhini and catarhini. According to Milner (1905), the axillary artery is dervied from the seventh or the ninth intersegmental artery in most of mammals, and an artery originating from the eighth intersegmental artery is rare. The artery in the present case may be non-differentiated in the sense that it has the vertebral artery derived from the sixth intersegmental artery and the deep cervical arteries originating between the sixth, seventh and eighth segments. Considering the comparative anatomy of the brachial plexus (Miller, 1934), there is a tendency for an increase in the number of radices and a more complicated plexus in higher vertebrates. Even platyrhini have the superior, middle and inferior trunks, as well as the posterior bundle as in humans. As in the present case, in some reptiles and monotremes the most inferior radix has no relationship with the posterior bundle. In summary, in the present case the anomaly of the muscle, vessels and nerves in the cervical region is hypothesized as maintaining a primitive morphological stage. V. Functional anatomy The human scalenus anterior functions mainly as a support and an elevator of the thorax. Considering the muscle from the relative position of origin and insertion, there is high mechanical efficiency when the origins is high and the insertion is more ventral. On the other hand, in tetrapedal animals with a horizontal body axis, the main function of the scalenus anterior is depression of the cervical region, thus the reverse relationship of the origin and insertion, because forelimbs support the vertebral column via the thorax. As gravity aides this function, further development of this muscle is not very necessary (Fig. 6). So, the importance of this muscle increases, as forelimbs become free from the support of body weight, during the evolutionary process of primates, typically shown in the upright posture of humans. Actually, the scalenus anterior is present behind the brachial plexus in prosimians, while the origin reaches the upper cervical vertebrae in higher primates (Forster, 1916). Consequently, it can be suggested that the mechanical efficiency of the scalenus anterior in question, which originates from the fifth and the sixth cervical vertebrae and inserts more dorsally, is less than that in the typical case. However, in the present case the muscle, vessels and nerves did not hinder normal functions, because there were no complaints of unusual feelings in the neck or left arm.

11 Scalenus Anterior Behind Subclavian Artery 239 Fig. 6. The working direction of the scalenus muscles in monkeys (left) and that in humans (right). Acknowledgements The author is very grateful to Prof. T. Yohro and Dr. M. Yoshioka of Faculty of Medicine, University of Tokyo for their critical advice for the manuscript. I would like to express my appreciation to Prof. T. Sato of Tokyo Medical and Dental University, Faculty of Medicine for his many helpful suggestions throughout this study. Also, the author wishes to thank the members of Department of Anatomy, University of Tokyo for their valuable discussion. References 1) Adachi, B.: Das Arteriensystem der Japaner. Bd. 1: Kyoto, ) Arakawa, H.: Zum Plexus brachialis der Japaner. Mie Med. J., 3: , ) Dolgo-Saburoff, B.: Zur Frage Uber die Lagebeziehungen zwischen der A. subclavia und der Scalenusmuskulatur beim Menschen. Anat. Anz., 76: , ) Eisler, P.: Die Muskeln des Stammes (Handbuch der Anatomie des Menschen. von K. V. Bardeleben, II Bd., II Abt., I Tell), G. Fischer, Jena, ) Forster, A.: Beitrage zur Morphologie des Scalenussystems und des M. sterno-costalis. Zeitschr. Morph. Anthro., 19: , ) Hirasawa, K.: Uber den Plexus brachialis. I. Mitteilung Die Wurzeln des Plexus brachialis. Acta Scholae Mediciitalis Universitatis Imperialis in Kioto, 11: , ) Kaibel, F. and Mall, F.P.: Handbuch der Entwicklungsgeschichite des Menschen. Bd. 1, S. Hirzel, Leipzig, ) Kaibel, F. and Mall, F.P.: Handbuch der Entwicklungsgeschichte des Menschen. Bd. 2, , S. Hirzel, Leipzig, ) Kanda, S.: Muscle and Artery system of the Japanese. In Anthropology, 6: The Japanese II, 13-61, (in Japanese) 10) Miller, R.A.: Comparative studies upon the morphology and distribution of the brachial plexus. Amer. J. Anat., 54: , ) Miller, R.A.: Observations upon the arrangement of the axillary artery and brachial plexus. Amer. J. Anat., 64: , ) Miiller, E.: Beitrage zur Morphologie des Gefasssystems. II Die Armarterien der Saugetiere. Anat. Hefte, 27: , ) Serizawa, M.: On the m. scalenus anterior in adult Japanese. Nippon Medical School J. 35: , (in Japanese) 14) Unehira, M. and Murakami, T.: Relation of the roots of the brachial plexus to the scalenus muscles. Acta Anatomica Niopponica 45: (8-9), (in Japanese) 15) Yamada, M.: Dissection Guide. 6th ed. 273pp, (in Japanese)

12 240 N. Inuzuka Abbreviations Aa : wdllery artery Aca : ascending cervical artery Ach : posterior circumflex humeral artery Acp : deep cervical artery Acs : circumflex scapular artery Ais : highest intercostal artery As : subclavian artery Ata : thoracoacromial artery Atd : thoracodorsal artery Ati : internal thoracic artery Av : vertebral artery C5-8 : 5th-8th cervical nerves Co 1 : first thoracic rib Fl : lateral cord of the brachial plexus Fm : medial cord of the brachial plexus Fp : posterior cord of the brachial plexus 0 : gravity Is6-9 : 6th-9th intersegmental arteries Msa : scalenus anterior muscle Msm : scalenus medius muscle Na : axillary nerve Nm : median nerve Nmc : musculocutaneus nerve Np : phrenic nerve Nr : radial nerve Nu : ulnar nerve Ti : first thoracic nerve Tc : costocervical trunk Ti : inferior trunk Tm : middle trunk Ts : superior trunk Tt : thyroid axis Vc6 : sixth cervical vertebra Vv : vertebral vein