THE HEART, 31 
as to show its structure. The ventral walls of the truncus arteriosus, and of 
the auricles and ventricle have been removed. (From a drawing by Mr. 
Hurst.) 
A, auriculo-ventricular aperture and valve: B, aperture leading from 
ventricle to truncus arteriosus, with its valve: C, left carotid arch: C’, style 
passed down right carotid arch into the truncus arteriosus: LA, left auricle: 
P, left pulmo-cutaneous arch: P’P’, style passed down right pulmo-cutaneous 
arch into the truncus arteriosus: PV, opening of pulmonary vein into left 
auricle: RA, right auricle: §, left systemic arch: S’, style passed down right 
systemic arch into the truncus arteriosus: SV, opening from sinus venosus 
into right auricle: V, ventricle. 
Having completed the dissection of the blood vessels, remove the 
heart completely, and dissect tt carefully under water so as to dis- 
play vts structure more fully. 
a. Cut open the ventricle from the ventral surface: note tts 
thick spongy walls, and the small size of its central cavity. 
b. Pass a seeker from the ventricle 
i. Into the truncus arteriosus, 
ii, Into the right and left auricles. 
C. Open the auricles from the ventral surface ; wash out the 
contained blood, and note the very thin wter-auricular 
septum diiding the right and left auricles from one 
another. 
d. Open the sinus venosus, and pass a seeker from wt into the 
right aurecle. 
e, Trace carefully the pulmonary vein, and see its opening into 
the left auricle. 
f. Cut away the ventral wall of the truncus arteriosus with fine 
scissors, to see the valves in rts wmterior. 
g. Cut across the aortic arches just beyond the division of the 
truncus into right and left branches, and note that though 
each branch is apparently a single vessel, rts internal 
cavity vs really divided into three vessels corresponding to 
the three aortic arches. Pass bristles down these aortic 
arches, and note the points at which they severally open 
into the truncus arteriosus. 
