302 



THE VASCULAR SYSTEM 



after, valves occur at intervals in the subclavian, axillary, and 

 other veins of the upper limb. 



The external jugular vein, as a general rule, possesses a 

 competent valve as it crosses the sterno-mastoid muscle. 

 Since no valves intervene between this point and the right 

 atrium (Fig. 108), venous pulsation may, under favourable 

 conditions, be observed in the external jugular of a perfectly 

 healthy subject. 



DEVELOPMENT OF THE HEART. In the young embryo, the 



Ext. Jug. Vein 



Int. Jug. Veins 



Ext. Jug. Vein 



Subclavian V 



FIG. 108. Diagram to show the positions of the valves on the 

 tributaries of the Superior Vena Cava. 



heart consists of a contractile tube, which is separated into 

 different parts by circular constrictions (Fig. 109). The large 

 veins open into the sinus venosus, which is placed at the 

 caudal (or posterior) end of the tube, and it pumps the blood 

 headwards into the atrium. The ventricle lies in front of the 

 atrium and conveys the blood to the truncus arteriosus, which 

 is the most cephalad, or anterior, part of the primitive tubular 

 heart. At this period of development, the heart receives its 

 nerve-supply from the sympathetic system, and as a result the 

 adult atria are supplied from a lower segment of the spinal 



