Dissection of Common Rat 199 



points does the arrangement of these vessels differ from that 

 in Man, where the external jugular runs parallel with the 

 posterior border of the sterno-mastoid muscle as far as its at- 

 tachment to the clavicle, and then perforates the deep fascia 

 dorsad of the clavicle to join the sub-clavian vein ; and the 

 internal jugular is the chief vein from the head. 



Dissection of Phrenic Nerves. The Student should 

 now remove the mass of Thymus gland that hides the 

 continuation of the jugular and subclavian veins in their 

 course towards the Heart ; and be careful not to cut away 

 the pericardium with it, nor injure the vessels covered 

 by it. 



To find the left Phrenic nerve, he will first note 

 the curve made by the descending cava round the first rib, 

 and the crescent it forms immediately afterwards, of which 

 the second left rib might form the centre. If then he 

 carefully dissects on the concave side of the crescent, he 

 will find close adherent to the vessel, at a point just oppo- 

 site the second rib, a thin white cord, which is the left 

 Phrenic nerve. This he should trace back nearly in a straight 

 line ventrad of the root of the single-lobed left lung, by the 

 side of the pericardium, between it and the mediastinal 

 portion of the pleura ; and by the left side of the fourth lobe 

 of the right lung, to the diaphragm. The right Phrenic 

 nerve may be traced in the same way, from the internal as- 

 pect of the concavity of the crescent of the right descending 

 cava, along the outer side of its posterior termination, across 

 the roots of the right lung, and along the course of the ascend- 

 ing or inferior cava to the diaphragm. These diaphragmatic 

 or Phrenic nerves take origin from the posterior cervical 

 nerves, and are therefore spinaly not cranial nerves. 



The existence of a left vena cava descendens is again a point 

 to be noted, as different from the arrangement of vessels 

 usually found in Man, where the left jugular and subclavian 



