skin so as to throw back a flap of body wall on each side; the left 

 one should show the anterior abdominal vein on its exposed 

 surface. 



e. \Yith forceps raise the sternum and carefully cut off the 

 fibrous bands seen passing to soft organs dorsal to it ; with strong 

 scissors cut through the sternum and other hard parts on the 

 median line carefully holding it up away from the soft parts dorsal 

 to it. Turn each half outward and pin firmly; pin the anterior 

 limbs out at full length. 



d. The liver is conspicuous, forming a large brown mass with 

 the pericardia! sac just anterior to it. 



f. Carefully cut away the membranous pericardium to expose 

 the heart ; then with great care clean off bit by bit the tissue covering 

 the vessels at the anterior end of the heart. 



g. ( i ) Xote the firm conical posterior portion of the heart, the 

 ventricle. (2) The cylindrical truncns artcriosiis arises from the 



m> 



right side of the base or anterior end of the ventricle and passes 

 obliquely forward to divide into two large branches. (3) The 

 atrium forms a thin walled sac dorsal to the truncus and anterior 

 to the ventricle (it is divided internally into two auricles). (4) 

 The sinus venosns can be seen by carefully raising the ventricle to 

 one side ; it forms a thin sac dorsal to the ventricle and atrium and 

 receives three large veins (two anterior or superior venae cavae 

 and one large posterior or inferior vena cava. } Two pulmonary 

 veins open into the left auricle by a single opening. ( 5 ) Each 

 branch of the truncus divides into three arteries, the most anterior 

 is the carotid, the most posterior the plumino-cutaneous and the mid- 

 dle one the systemic aortic arch which unites with its fellow dorsal to 

 the heart to form the dorsal aorta. 



Make a diagram of the heart and vascular trunks. 



h. Pulsations of the heart. Observe : ( i ) A regular sequence 

 of contraction and dilation. ( 2 ) The atrium contracts, then the 

 ventricle, and immediately after the truncus. ( 3 ) On raising the 

 ventricle, the sinus venosus can be seen to contract before the 

 atrium. The contraction proceeds in the same order as that fol- 

 lowed by the blood in passing through the heart. 



i. Anterior to the heart note the broad flat transverse mylo- 

 hyoid muscle through which can be seen the long first vertebral 

 nerve or hypoglossal. Note also the hard portuberant larynx and 

 on each side of this a small soft body, the thyroid gland. 



]. Posterior to the heart, note the following viscera: (i) The 



59 



