466 Description of a 4-mm. Human Embryo 
into the second and third arches of each side. The connection between 
these last two pairs of arches and the aortic stem is very slender, com- 
pressed laterally, as though the arches had grown from the dorsal aorta 
and had only just united with the ventral aorte. Of the fourth and 
fifth aortic arches the only trace is the rim of the cup-shaped part of the 
ventral aorta, shown in the figure below the bend of the third aortic 
arch. No branches come from the dorsal aorte in the position of these 
fourth and fifth arches. The dorsal aorte are joined in a single median 
vessel for a considerable distance along the back; there is no trace of 
carotid branches from the first arches toward the head. 
MODEL OF HEART. 
An anterior view of the model of the heart is given in Fig. 9, the 
model is seen slightly from the right side. The most anterior portion 
is the large rounded aortic bulb (B. ao.), cut off just below its division 
into aortic arches as shown in the model of the pharynx. Continuous 
with this is the ventricle (Vent.) a large single chamber, beginning at a 
constriction marked by a groove on the outside (Gr.), extending down- 
ward on the left side, turning on itself to form the apex of the heart, and 
then tapering upward to merge with the aortic bulb. The walls of the 
ventricle are thick, smooth externally, but internally very irregular, with 
deep pockets ined by endothelium leading from the main chamber, rep- 
resenting the sinusoids of the heart. In the aortic bulb the walls be- 
come smooth internally; there is no sign of division into right and left 
ventricles. The left auricle (Lt. au.) is placed above the ventricle, 
marked from it externally by the groove. Its walls are thin and folded, 
probably by pressure, and internally the endothelium is smooth, with no 
pockets nor trabecule. The left and right auricles are joined near their 
upper ends anteriorly by a prominent ridge, enclosing a cavity, which 
extends downward, compressed antero-posteriorly, and opens by a nar- 
row channel into the ventricle to the left of the median line. Above 
this connection the right auricle rises dorsally to a rounded peak, not 
quite so high as the top of the left auricle, from which it is separated 
by the intestine (/n.), but, unlike the left auricle, the right extends far 
below the auriculo-ventricular canal as a spacious pouch, reaching almost 
to the level of the apex of the ventricle, and occupying a position behind 
and to the right of the aortic bulb and the lower part of the ventricle. 
Here, as in the left auricle, the walls are thin, smooth on both external 
and internal surfaces, and thrown into great folds. If this auricle were 
distended it could certainly contain a larger volume of blood than the 
