24 



14. Cut away the basi-branchial plate on the floor of the mouth; carefully 

 dissect off the mucous membrane from the roof of the mouth, and examine 

 the efferent branchial arteries. 



Observe that the efferent branchial arteries unite to form the dorsal aorta. Note 



(a) That the two branches from the lamellae of the 1st gill cleft unite with two 



branches from the lamellae of the 2nd gill cleft to form the first efferent 

 artery. 



(b) That the two branches from the 3d gill cleft form the second efferent artery. 



(c) That the third efferent artery is formed of three branches — two from the 



4th and one from the 5th gill cleft. 



15. Note— 



(a) The vertebral arteries springing from the 1st pair of efferent arteries and 

 passing through the anterior vertebral plate to the brain and spinal cord. 



(6) The common carotid arteries arising from the anterior branch of the first 

 efferent arteries just before they emerge from the first gill cleft. Trace 

 them forwards, and note that each gives off a small spiracular artery, and 

 then bifurcates to form 



(1) An external carotid supplying the jaws, etc. 



(2) An internal carotid — the right and left internal carotids meet in 



middle line and pass through a foramen in the floor of the skull 

 to supply the brain, etc. 

 (c) The brachial arteries springing from the aorta in front of the third efferent 

 arteries. 



16. Trace the dorsal aorta backwards, and note — 



(a) The origins of the cceliac and superior mesenteric arteries. 



(6) The spermatic arteries supplying the testes (ovarian if a female). 



(c) The inferior mesenteric artery supplying the rectum and rectal gland. 



(d) The renal arteries supplying the kidneys. 



(e) The iliac arteries. 



(/) The continuation of the aorta along the haemal canal of the vertebral 

 column as the caudal artery. 



17. Slit up the conus arteriosus, and note the three longitudinal rows of valves, 



each row consisting of five semi-lunar valves, the fifth valve of each row being 

 largest. 



18. Carefully remove the heart, including the sinus venosus, from the pericardial 

 chamber, cut open the auricle and ventricle, and note — 



(a) The delicate walls of the auricle. 



(b) The thick walls of the ventricle and the muscular columnae carneae 



strengthening them. 



(c) The large aperture between the sinus venosug and the auricle, and the 



two folds forming the sinu-auricular valve. 



(d) The two folds forming the valve of the auriculo-ventricular aperture — ^look 



for the small cusps in the triangular spaces left between the large folds. 



J. The sympathetic nervous system. 



1. Note the series of sympathetic ganglia running along each side of the 

 vertebral column (a large ganglion lies close to the inner side of each 

 Cuvierian sinus). Observe that the ganglia are connected by commissures 

 to each other, and to the cerebro-spinal nerves. 



