APPENDIX. 445 



the left side of the ventricle, under the fleshy ridge which projects 

 from the superior wall and hase into the compartment from which 

 issue the two aortae, and with which the pulmonary compartment 

 communicates. It passes, however, under the fleshy ridge or sep- 

 tum (septum hetween venous and arterial ventricular compartments) 

 towards the arterial compartment into which the hlood of the left 

 auricle begins to pass, a little later than that of the right into the 

 right compartment. The passage of the venous hlood from right 

 to left is impeded by the spongy arrangement of the anterior or 

 under part of the ventricle. In consequence, therefore, of first, the 

 prior entrance of the venous hlood, and of the course of least resist- 

 ance into the venous compartment of the ventricle given it by the 

 arrangement of the right auriculo-ventricular valve ; second, the 

 secondary entrance of the arterial blood of the left auricle, and its 

 course of least resistance given by the left auriculo-ventricular valve ; 

 and, third, the retardation of the passage of the venous blood to 

 the left side by the spongy texture ; time is afforded for a sufficient 

 quantity of mixed venous blood passing into the pulmonary artery 

 by the prior contraction of the right side of the ventricle. After 

 the closure of the pulmonary compartment and its complete con- 

 traction, the pulmonary artery begins to react, while the left side of 

 the ventricle continuing to contract, black blood continues to flow as 

 it did from the first into the aorta, but with more admixture of 

 arterial blood, until the last blood, perhaps entirely arterial, alone 

 passes through them. 



Ma;/ IS. — I removed by means of a trephine a circle of bone 

 over the pericardium of a small tortoise of the same species. The 

 small size of the heart enabled me and John Arthur to discriminate 

 very distinctly the prior or initial contraction of the right auricle, 

 and also the commencement of ventricular contraction in the right 

 side, its longer continuance on the left. The portion of the ven- 

 tricle which continues longest to contract is the left angle of the 

 ventricle, which receives the blood of the left auricle, as shown by 

 the scarlet tint of both. The black blood of the right auricle was 

 seen, on entering the ventricle, to occupy in the first place the right 

 side of the cavity, and then to disperse itself towards the left, 

 on the sternal wall of the chamber, the posterior part of the left 

 side of tin', chamber being occupied by red blood. 



