CERVICAL PLEXUS. 81 



On the left side the nerve crosses over the first part of the subclavian 

 artery. 



The branches communicating with the descendens noni are two in num- 

 ber. One arises from the second, and the other from the third cervical 

 nerve ; they are directed inwards over the internal jugular vein, and com- 

 municate in front of the carotid sheath with the descending muscular 

 branch (descendens noni) of the hypoglossal nerve. Sometimes these 

 nerves pass under the jugular vein. 



Muscular branches are furnished to the anterior recti muscles ; they 

 arise from the loop between the first two nerves, and from the trunks of 

 the other nerves close to the intervertebral foramina. 



Some connecting branches pass from the loop between the first two 

 nerves, and unite with the sympathetic and some cranial nerves near the 

 base of the skull: these will be afterwards described. 



EXTERNAL OR POSTERIOR SERIES. Muscular branches are given 

 from the second nerve to the sterno-mastoideus ; from the third nerve to 

 the levator anguli scapulae ; and from the third and fourth nerves to the 

 trapezius. Further, some small branches supply the substance of the mid- 

 dle scalenus. 



Connecting branches with the spinal accessory nerve exist in three 

 places. First, in the sterno-mastoid muscle; next, in the posterior trian- 

 gular space ; and lastly, beneath the trapezius. The union with the branches 

 distributed to the trapezius has the appearance of a plexus. 



The COMMON CAROTID ARTERY is the leading vessel for the supply of 

 blood to the neck and head (fig. 16, '). The origin of the vessel differs on 

 opposite sides of the body, beginning at the lower part of the neck on the 

 right, and in the thorax on the left side. 



The right vessel commences opposite the sterno-clavicular articulation 

 in the bifurcation of the innominate artery, and ends at the upper border 

 of the thyroid cartilage by dividing into the external and internal carotid. 

 The course of the artery is along the side of the trachea and larynx, gradu- 

 ally diverging from the vessel on the opposite side in consequence of the 

 increasing size of the larynx; and its position will be marked by a line 

 from the sterno-clavicular articulation to a point midway between the 

 angle of the jaw and the mastoid process. 



Contained in a sheath of cervical fascia with the internal jugular vein 

 and the pneumo-gastric nerve, the carotid artery has the following connec- 

 tions with the surrounding parts: As high as the cricoid cartilage the 

 vessel is deeply placed, and is concealed by the common coverings of the 

 skin, platysma, and fasciae, and by the muscles at the lower part of the 

 neck, viz., sterno-mastoid (sternal origin), sterno-hyoid, omo-hyoid, and 

 sterno-thyroid. But above the circoid cartilage to its termination the 

 artery is less deep, being covered only by the sterno-mastoid with the 

 common investments of the part. The vessel rests mostly on the longus 

 colli muscle, but close to its ending on the rectus capitis anticus major. 

 To the inner side of the carotid lie the trachea and larynx, the ojsophagus 

 and pharynx, and the thyroid body, the last overhanging the vessel by the 

 side of the larynx. Along the outer side of the carotid sheath is a chain 

 of lymphatic glands. 



Veins. The large internal jugular lies on the outer side and close to the 



carotid at the upper end, but separated from it below by an interval of 



about half an inch : on the left side the vein is over the artery below, as 



will be afterwards seen. One or two upper thyroid veins and their branches 



6 



