DE. J. MUEIE ON THE OEGANIZATION OP THE CAAING WHALE. 



267 



other words, the valve consisted of two segments, the delicate free margins of the 

 segments being supplied with numerous fine chordae tendinese attached to the rudi- 

 mentaiy musculi papillares of the right ventricle. From this it follows that the 

 supplementaiy mitral valve within the septal segment of the tricuspid (this valve 

 within a valve) appears to be acted upon by the musculi papillares and by the blood, 

 precisely in the same manner as the mitral and tricuspid valves themselves ; i. e. it is 

 opened during the diastole of the heart, and closed during its systole. 



By reference to figures 52 & 53 a better conception of the position of the perforation 

 and its relation to the tricuspid will be gathered than by entering into minutiae verbally. 

 I may remark, however, that this ancillary valve, whatever its homological significance, 

 appears to me to be to the valvular system of the heart, what the rete mirabile is to 

 the vascular system generally. One may regard it as a tendency in nature to repeat 

 herself, a something superadded vrithout apparently any special end to be served. It 

 may be imagined to weaken rather than strengthen the action of the tricuspid ; on the 

 other hand, it may be a kind of safety-valve'. From its anatomical structure, its 

 physiological influence can only be conjectured. The rarity of such an instance shows 

 that it is not a necessity to Cetacean circulation. 



Fig. 4, 



The arch and great arteries of the Pilot Whale, after Turner, from the Joum. of Anat. ii. p. 67. 



A, aorta ; P, pulmonary artery ; D, ductus arteriosus ; a, carotis cerebralis, the diminution in calibre of this artery in the course 

 of its ascent is not sufficiently represented in the figure ; b, carotis facialis ; c, subclavia ; d, cervico-oc<;ipalis ; e, thoracica 

 posterior dextra ; /, transTersalis colli ; g, mammaria interna ; A, thoracica posterior sinistra. 



' See a paper by T. King " On the Safety-valve Functions in the Eight Ventricle of the Heart," Guy's Hosp. 

 Eeports, vol. ii. 

 VOL. viiL — PART IV. February, 1873. 2 q 



