GENERAL BIOLOGY 



g. 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 protuberant larynx and 

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



2. CIRCULATORY SYSTEM. 



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. 



a. Heart. 



(i) Note the firm conical posterior portion of the heart, the 

 ventricle. (2) The cylindrical truncus arteriosus arises from the 

 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 venosus 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 

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

 veins open into the left auricle by a single opening. 



b. 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 followed by the blood in passing through 

 the heart. 



c. Arteries (the efferent vessels). 



The blood is carried through the common truncus arteriosus 

 which gives rise to a right and left subdivision. Each of these 

 divides into three branches derived from three embryonic arches : 



1 i ) The most anterior branch or arch, the carotid, bears near 

 its point of origin a pinkish glandular enlargement, the carotid 

 gland. The common carotid divides into (a) the external carotid 

 situated near the median line but distributed to the superficial 

 tissues of the head, and (b) the internal carotid which passes into 

 the cranial cavity to supply the brain and sense organs (eye, in- 

 ternal ear). 



(2) The middle or systematic arch unites with its fellow on the 



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