330 



AVES. 



distinct cavities, wluch have the same relations 

 to each other, and impress the same course 

 on the circulating fluid.* 



The form of this viscus is always that of a 

 cone, sometimes wide and short, as in the 

 Ostrich and Crane ; sometimes more elongated, 

 as in the Emeu (Jig- 167) and Vulture; or 

 still more acute, as in the Curlew, Common 

 Fowl, &c. 



Its situation is more anterior and mesial than 

 in Mammalia, and its axis is always parallel 

 with the axis of the trunk. It is not contained 

 with the lungs in an especial cavity, but its 

 apex is lodged between the lobes of the liver ; 

 the diaphragm not being so far developed as 

 to separate the chest from the abdomen. 



As the lungs are confined to the dorsal part 

 of the chest, the whole of the anterior surface 

 of the pericardium is exposed when the sternum 

 of the bird is removed. The pericardium is thin, 

 but of a firm texture, and adheres by its ex- 

 ternal surface to the surrounding air-cells. It 

 is of considerable size, and commonly prolonged 

 for some way between the lobes of the liver. 



The auricles of the heart in Birds have not 

 externally such distinct appendices as in Mam- 

 mals. The right auricle is much larger than 

 the left; it is more distinctly divided internally 

 into a sinus (d, fg. 167) and auricle proper 



Fig. 167. 



Heart of the Emeu, 



than in Mammals, and these parts are separated 

 by a more complete valvular structure ; in which 

 respect Birds bear a closer analogy to Reptiles. 

 Three veins terminate in the sinus, there 

 being in Birds always two superior cava?, 

 as in Reptiles. The right superior cava (), 

 which returns the blood from the right wing 

 and right side of the neck, terminates in 

 the upper and anterior part of the sinus ; the 

 left superior cava (b, b) winds round the pos- 

 terior part of the left auricle to open into the 



* The blood of Birds differs from that of the 

 other Vertebrate classes in the greater number of 

 globules, and from that of Mammalia in their 

 form, which is oval iustcad of round. See BLOOD. 



lower part of the sinus; just before its termination 

 it receives the coronary vein, so that this does 

 not open separately into the auricle as in most 

 Mammalia.* The inferior cava (c) terminates 

 in the sinus just above the orifice of the left su- 

 perior cava, and a semilunar valvular fold (//), 

 analogous to that of the coronary vein in man, 

 is extended forwards between these orifices so 

 as to separate them, and afford a protection to 

 the mouth of the left superior cava, in addition 

 to that which it derives in common with the 

 other veins from the larger valves at the mouth 

 of the sinus. 



The disposition of the valves between the 

 sinus and auricle seems more especially des 

 tined to prevent regurgitation into the sinus, 

 when the pulmonary circulation may be im- 

 peded, rather than to impress any definite 

 course on the current of blood flowing through 

 the auricle, as Cuvier supposes. A strong 

 oblique semilunar muscular fold (g) commences 

 in the Emeu by a band of muscular fibres 

 running along the upper part of the auricle, 

 and expanding into a valvular form extends 

 along the posterior and left side of the sinus, 

 terminating at the lower part of the fossa ovalis 

 (). A second semilunar muscular valve ( /), 

 of equal size, extends parallel with the preceding 

 along the anterior border of the orifice of the 

 sinus, its lower extremity being fixed to the 

 smooth floor of the auricle, its upper extremity 

 being continued into a strong muscular column 

 running parallel to the one first mentioned 

 across the upper and anterior part of the 

 auricle, and giving oft' from its sides the greater 

 part of the musculi pectinati. From this struc- 

 ture it results that the more powerfully the 

 musculi pectinati act in overcoming the ob- 

 stacle to the passage of the blood from the 

 auricle to the ventricle, the closer will the valves 

 be drawn together, and the stronger will be 

 the resistance made by them to the regurgitation 

 of the blood from the auricle into the sinus. 

 The parietes of the auricle in the interspaces of 

 the muscular fasciculi are thin and transparent, 

 consisting in many parts only of the lining mem- 

 brane of the cavity and the reflected layer of the 

 pericardium blended together. The/bsz ovalis 

 ( i) is a deep depression situated behind the pos- 

 terior semilunar valve, which, we may observe, 

 bears nearly the same relation to the fossa as 

 the annulus ovalis in the human heart. The 

 membranous septum closing the foramen ovale 

 is complete and strong, but thin and semi- 

 transparent. The appendix auricula (x) is the 

 most muscular part of the cavity ; it does not 

 project freely in front of the great vessels 

 arising from the ventricles, but is tightly tied 

 down to them by the reflected layer of the 

 pericardium. The auriculo-ventricular orifice 

 is an oblique slit (k,jig. 169 ; a bristle is passed 

 through it in Jig- 167). The manner in which re- 



* In those Mammalia which approach nearest (o 

 the oviparous vertebrata, as the Monotremata and 

 Ularsupiata, there are always two superior cavae, as 

 in Birds and Reptiles ; a similar structure obtains 

 in some of the Rodentia, as the Porcupine ; and 

 also occurs in the Elephant. In all these cases we 

 have found that the coronary vein terminates in 

 the left superior cava. 



