226 PROFESSOR MARSHALL AND EDWARD J. BLES. 



The lungs themselves have increased in size, and form a pair of 

 hollow sacs with thick walls, extending back alongside the oesophagus 

 to about the middle of the length of the head kidneys. 



3. The Heart and Pericardial Cavity. 



Concerning the pericardial cavity at this stage, we have nothing 

 special to say. It communicates freely with the ccelom both dorsally 

 and ventrally. 



The heart is more completely twisted on itself than before, and the 

 several divisions more distinctly marked off from one another. 



The sinus venosus (Fig. 12, H S) is a wide transverse vessel 

 running across the hinder and dorsal part of the pericardial cavity ; 

 it receives at its outer ends the Cuvierian veins, which are now of 

 great size (Figs. 11 and 12, V D); and into its posterior wall open 

 the hepatic veins (V H). In the middle of the anterior wall of the 

 sinus venosus is a large round aperture leading into the auricle. 



The auricle (Figs. 11 and 12, H A) is a large, thin-walled, globular 

 sac, lying in the dorsal part of the pericardial cavity, slightly to the 

 right side : there is as yet no trace of the interauricular septum. The 

 auriculo-ventricular aperture is wide, and lies almost horizontally, 

 its actual position, however, varying according to the condition of 

 contraction of the heart. 



The ventricle lies below and to the left side of the auricle. Its 

 wall is no longer of uniform thickness, but is strengthened by a 

 reticulum of muscular fibres formed in its substance. These fibres 

 are best developed in the posterior and ventral region, which will 

 afterwards form the apex of the ventricle. 



The truncus arteriosus arises from the ventricle at the right hand 

 dorsal corner of its anterior face, as shown in Fig. 11, HM; it then 

 runs forwards, upwards, and slightly to the left, reaching the anterior 

 wall of the pericardial cavity in the median plane (Fig. 12). The 

 walls of the truncus arteriosus are still thin, and its aperture of 

 communication with the ventricle is a narrow one. 



In front of the pericardial cavity the truncus divides at once, as 

 before, into right and left branches (Fig. 12), which run outwards 

 and slightly backwards. Each branch quickly divides into two, of 

 which the anterior is the afferent vessel for the first branchial arch 

 and its external gill, while the posterior, after a short course 



