The conzmunicating branches have been al ready partly described in the consideration of the formation of the plexus ; and are formed by different filaments, which are connected with the trunk of the sympathetic, its upper and middle cervical ganglion, as also with the par vagum and lingual. The internal deep branch is represented by the communicans noni or internal descending cervical, It takes its origin principally from the descending di vision of the second cervical, and having re ceived a filament from the first cervical, coines down the neck external and posterior to the internal jugular vein. At the middle or lower third of the neck, it describes a curve ; the concavity of which looks upwards, and com municates with the descending branch of the lingual (descendens noni), by winding in front of the internal jugular vein. This nerve is subject to considerable variation, bifurcating, occasionally, before communicating with the descendens noni, and giving off' now and then one or two delicate filaments to the same muscles, usually supplied by the latter nerve : viz. the sterno-hyoid, sterno-thyroid, and omo-hyoid. The external communicating branches are represented by a rather larger anastomotic branch, which communicates at au acute angle with the spinal accessory ; and by the muscular branches. These accompany the spinal accessory, communicate more or less with it, and are distributed to the trapezius, le vator anguli scapulm, the rhomboideus rninor, and upper part of the rhoniboideus major.
The phrenic nerve (diaphragmatic, internal respiratory) appears at the lower and anterior part of the fourth cervical nerve of which it appears the continuation. It receives, how ever, some accessory filaments from the third and fifth cervical, which exist either as single or plexiforin twigs, or are occasionally absent. The secondary sources of origin are, in fact, subject to considerable variation. It is di rected along the anterior edge of-the scalenus antieus, inclining slightly inwards between the subclavian artery and vein, before entering the superior opening of the thorax. It passes behind and outside the carotid artery and jugular vein, and communicates with the fifth, sixth, and, occasionally, with the seventh cervical and pneumogastrie nerves, and in variably with the sympathetic. The exact points of communication of these different nerves is by no means determinate .; some times taking place in the neck ; at others, in the upper part of the chest. It crosses the direction of the internal mammary artery, and reaching the anterior mediastinum, glides down in front of the.root of the lung between the pericardium and inner aspect of the former, and terminates in the diaphragm. In its course within the chest, it gives several fila ments to the remains of the th3naus gland ; some very minute twigs of communication with the superior cardiac plexus ; and re ceives, occasionally, a very delicate filament of communication, coming down obliquely, from the descendens noni : on reaching the diaphragm, the nerve divides into a series of superior and inferior filaments ; the former, long and diverging froin each other, enter the upper surface of the muscle, having first passed for some distance between the muscle and the pleura covering it; the latter per forate the muscle, diverge, and run for some distance between the muscle and peritoneum, and enter its under surface.
The right phrcnic is shorter and more ver tical in direction, and more anterior in its position than the left, being directed in the upper part of the chest, along the vena cava superior. Several of its internal terminal filaments pass behind the vena cava inferior, coinmunicate with the left, and end in the cceliac plexus ; a few, however, communi cate, also, with some twigs of the pneumo gastric.
The left phrenic turns over the apex of the heart; and, besides its general distribu tion, gives filaments to the crura of the dia phragm, anastomosing filaments to the solar and cceliac plexus, and some communicating branches to the opposite nerve.
The anterior branches of the four inferior cervical and first dorsal nerves are very largo, and form, therefore, a remarkable contrast to the four upper cervical, situated above them. They pass through the intervertebral foramina, between the two scaleni ; the eighth cervical passing between the foramen common to the last cervical and first dorsal vertebra. Having given off several filaments to communicate with similar filaments from the inferior and middle cervical ganglion, and some small twigs to the sealeni, the different branches unite together, so as to constitute the bra chial plexus; the first, communicating above with the fourth cervical, and sending a twig to the phrenie. The union of the different branches takes place in the following manner : — the anterior branches of the fifth and sixth descend obliquely outwards, and, after a course of about one or two inches, unite at an acute angle. Those of the eighth cervical and first dorsal, which are not so oblique in their direction, similarly unite; but a little more internally : this union taking place, oc casionally, 'between the scaleni, either pair of branches almost immediately bifurcating after their union. The trunk of the seventh passes distinct between the two upper and lower branches, as far as the lower border of the clavicle in the upper part of the axilla, and there bifurcates ; the upper part of the bifur cation being connected with the lower part of the bifurcation of the first united cord, and the lower with the upper of the last united cord. Secondary bifurcations and anasto moses take place at more or less acute angles, and thus the brachial plexus is constituted.