202 THE FROG— THE PLATANA 



B. Venous System and Nervous System [part). 

 1. Note:— 



(a) The heart within a thin pericardiixm, and consisting of the sinus venosus, 

 the two auricles (divisions of the atrium), the single ventricle, and 

 the conus arteriosus which is continued into the ventral aorta (the 

 " truncus ") which divides into two branches. 



Remove the pericardium from the heart and tUt the apex of the 

 heart forwards, noting the dorsal and roughly triangular sinus venosus 

 into which the right and the left superior venae cavse (or precavals) 

 and the single inferior vena cava (or postcaval) open. 

 (6) The veins passing from each side of the sinus venosus. Each superior 

 vena cava is forrned by the union of the following branches : — 



(i) The external jugular vein, formed by the union of the lingual 



vein from the tongue and the mandibular vein from the 



margin of the lower jaw. 

 (ii) The innominate vein, formed by the union of the internal 



jugular vein from the head and the subscapular vein from 



the back of the ai^m and shoulder, 

 (iii) The subclavian vein, formed by the union of the brachial vein 



from the fore-limb and the musculo-cutaneous vein from the 



muscles of the body and from the skin. 



[In the Platana the superior vena cava is formed by the following vessels : — 



(i) The external jugular vein, which passes under the mylo-hyoid 

 muscle, from the region of the tongue and mandible. 



(ii) The internal jugular vein, which runs on the under side of the 

 floor of the mouth, parallel to the exterrml jugular. 



(iii) The thick subscapular vein from the shoulder. As the internal 

 jugular and the subscapular veins join the precaval vein separ- 

 ately, there is no innominate vein. 



(iv) The subclavian vein, which is formed by the brachial vein from 

 the arm, a cutaneous vein from the skin, and a muscular vein 

 from the abdominal rnu^cles. The muscular and the cutaneous 

 veins are separate, consequently there is no musculo-cutaneous 

 vein.] 



(c) The hypoglossal (spinal nerve II) and the glosso-pharyngeal nerves, both 

 running alongside the lingual vein. \In the Platana the glosso-pharyn- 

 geal nerve runs along the inner side of the lower jaw, while the first spinal 

 nerve {corresponding to the hypoglossal of the Frog) curves inwards to 

 supply the mylo-hyoid muscle.] 



{d) The vagus nerve, supplying the heart and lungs and stomach. It 

 emerges just posterior to the glosso-pharyngeal, and closely foUows 

 the course of the internal jugular vein. [In the Platana it follows the 

 course of the subscapular vein.] 



Make an enlarged drawing showing the above parts. [See Fig. 67, A or B, 

 page 196.) 



