HEAD AND NECK OF THE HORSE 3 



the common carotid artery. Cavidal to the fifth vertebra the right 

 jugular is related to the carotid artery and the trachea, and the left 

 vein to the artery and the oesophagus. 



The following tributaries join the jugular at different points in its 

 course : — 



(1) The thyroid vein (v. thyreoidea) is a large vessel that joins the 

 jugular close to its commencement, and corresponds to an artery of the 

 same name that will be disclosed as the dissection proceeds. It will 

 be found later that this vein drains the pharynx and larynx in addition 

 to the thyroid gland. 



(2) Muscular, tracheal and cesophageal veins enter the jugular at 

 numerous points as it travels down the neck. 



(3) The ascevding cervical vein (v. cervicalis ascendens) is often 

 the last tributary, and sometimes joins the subclavian vein instead of 

 the jugular. 



(4) The cephcdic vein (v. cephalica), a superficial vein of the thoracic 

 limb, sinks into the triangular space (supraclavicular fossa) at the root 

 of the neck to join the jugular close to its termination. 



The small ramus colli of the seventh cerebral nerve appears between 

 the parotid gland and the inferior auricular muscle, is connected with 

 the second cervical nerve, and follows the dorsal border of the jugular 

 vein to supply the skin and cutaneous muscle of the neck. 



N. CUTAXEUS COLLI. — The cutaneous nerve of the neck is derived 

 from the second cervical, which pierces the brachio-cephalic muscle on a 

 level with the commencement of the jugular vein. One (or more) of 

 its branches is connected with the ramus colli of the seventh cerebral 

 nerve. Another branch runs horizontally forwards into the space 

 between the two halves of the mandible. 



Dissection. — As the jugular vein affords an excellent opportunitj' for 

 the study of the valves that are present in nearly all the veins of the 

 body, the vessel should be incised longitudinally. The valves are pocket- 

 like folds of the innermost coat of tlie vein and occur at fairly regular 

 intervals in pairs (occasionally three valves may occur at the same level). 

 Each valve has a convex margin attached to the wall of the vein, and a 

 free slightly concave border. The mouth of the pocket formed by the 

 valve looks towards the heart, so that a reflux of blood fills the pocket, 

 causes the valve to bulge into the interior of the vein, and thus occludes 

 the lumen of the vessel. Coinciding in position with the valve there is 

 a dilatation of the wall of the vein, with the result that the vessel, when 

 distended with blood, presents a beaded appearance. 



Cut across both sterno-cephalic muscles about the middle of the neck 

 and turn them aside. 



M. STERNOHYOIDEUS : M. STERXOTHYREOIDEUS. — These two 



muscles have a common origin from the manubrium of the sternum. 



