Frederic T. Lewis 219 
proceeds the shelf-like anlage of the atrio-ventricular wall and mitral 
valve. The left auricle passes caudad into a funnel-like prolongation, 
the pulmonary vein. As in-man, this is at first a single tube, but later 
is taken vp into the auricle so that its several branches enter by sepa- 
rate orifices. 
The left ventricle is cut off from the right by a trabecular partition, 
capped dorsally by a mass of dense mesenchyma which bounds the ill- 
named interventricular foramen. This foramen opens into the space 
a. b, ec, shown in a section of the heart on the right of the median septa, 
Plate IV. From here the blood may proceed either through the aorta or 
the pulmonary artery. The aorta and pulmonary artery are separated 
by a pair of folds united above, but distinct below. The fold on the 
left side, which has been cut away, passed over into the interventricular 
septum near b. The other extends along the right cardiac wall, ending 
in the tricuspid anlage. These folds fuse so as to connect a with b, 
and b with e. The only outlet for the persistent interventricular fora- 
men is then into the aorta (Born, 89, p. 339). The cut tissue near b 
is not involved in the division of the bulbus arteriosus just described. 
It represents a clump of trabecule passing from the right wall and sec- 
tioned just before uniting with the median septum. It is connected 
with the tricuspid trabecule. Instructive cross-sections at this level 
have been drawn by Hochstetter, 02, p. 55. 
The right auricle still has a single opening for. the ducts of Cuvier 
and the inferior vena cava. They unite in the sinus venosus which 
empties between its two valves. The valves are united above, forming 
the septum spurium. In man the septum spurium and a part of the 
left valve are used in closing the foramen ovale; parts of the right 
valve persist as the Eustachian valve and valve of Thebesius. 
Arteries.—The arterial system is represented in Plate III. The pul- 
monary artery leaves the heart by a single trunk, which divides into 
two arches of equal calibre, passing to the right and left aorte respec- 
tively. From the right arch a large stem, and from the left a slender 
one, unite and proceed ‘to the lungs. Only the left arch and its stem 
contribute to the adult pulmonary artery as described by Bremer, 02. 
The aorta also begins as a single trunk which bifurcates, forming 
right and left divisions. Beyond the bifureation the ventral section of 
the aorta continues to the tongue and jaw as the external carotid artery. 
External and internal carotid arteries are united by the carotid arch. 
A very short section of ventral aorta represents the common carotid 
artery. 
Proceeding caudad, the internal carotid artery passes into a slender, 
