CIRCULATING APPARATUS OP MAMMALS. 117 



canal, nearly always resemble those which have been described 

 in Man. (ANIM. PHYSIOL.) 



101. This is the case also with the Circulating apparatus, 

 and with the respiratory organs. The heart possesses, in every 

 instance, four cavities ; known as the two auricles, and the two 

 ventricles (Fig. 66). The lungs are always divided into an im- 



Superior Pulm. Pulmonary 



venacava. art. Aorta. artery. 



Pulmonary veins. <',. 

 Right auricle. 



Tricuspid valves. 



Inferior vena cava. 



-? Pulmonary veins. 



Left auricle. 



-- Mitral valve. 



Left ventricle. 



Right ventricle. '" 



Partition. 



Aorta. 

 FIG. 66. IDEAL SECTION OF THE. HUMAN HEART. 



mense number of very small cells, and do not allow the air to 

 pass from their interior into the different parts of the body, 

 as takes place amongst Birds. It is further to be remarked, that 

 the blood in this class of animals is always very rich in organ- 

 ised matter, and that its red particles are nearly always of a 

 circular form. 



102. The Mammalia are, of all animals, those which approach 

 the nearest to Man, in regard to their intellectual powers. But 

 in this respect they present the greatest differences amongst them- 

 selves, which have been elsewhere noticed (ANIM. PHYSIOL. 

 717) ; and if space would permit us here, it would be easy to 

 multiply the proofs of this inequality. The study of the habits 

 of the Mammalia would furnish us also with curious examples 

 of the various kinds of instincts given by Nature to these beings, 

 to supply the want of the more elevated faculties ; the most im- 



