B. I.] THE HISTOBY OF ANIMALS. 21 



many numerous veins at its junction with the intestines, 

 reaching from above downwards. This is the nature of the 

 oesophagus, trachea, and the parts of the abdominal cavity. 



CHAPTER XIV. 



1. THE heart has three cavities : it lies above the lungs, near 

 the division of the trachea. It has a fat and thick membrane, 

 by which it is united to the great vein and the aorta, and it 

 lies upon the aorta near the apex ; and the apex is placed in 

 the same situation in all animals which have a chest ; and in 

 all animals, whether they have or have not a chest, the apex 

 of the heart is forwards, though it often escapes notice by 

 the change of position in the parts when dissected. The 

 gibbous portion of the heart is upwards ; its apex is gene- 

 rally fleshy and thick, and there is a sinew in the cavities. 



2. In all other animals which have a chest the heart is placed 

 in the centre ; in man it is rather on the left side, inclining 

 a little from the division of the mammae towards the left 

 breast in the upper part of the chest ; it is not large ; its 

 whole form is not long, but rather round, except that the 

 extremity ends in a point It has three cavities, as I have 

 said. The greatest is that on the right, the least on the 

 left, the middle one is of intermediate size. They are all 

 perforated towards the lungs. It has both the two smaller, 

 and all of them perforated towards the lungs, and this is 

 evident in one of the cavities downwards from its point of 

 attachment. 



3. Near the principal cavity it is attached to the great 

 vein to which also the mesenterium is united, and in the 

 middle it is attached to the aorta. Passages lead from 

 the lungs to the heart, and they are divided in the same 

 way as the trachea, following the passages from the trachea 

 throughout the whole lungs, and the passages leading from 

 the heart are on the upper part. There is no passage which 

 is common to them both, but by their union they receive the 

 breath and transmit it through the heart ; for one of the 

 passages leads to the right cavity, and the other to the left. 

 We will hereafter speak of the great vein and the aorta in 

 the portion of our w r ork which treats of these parts. 



4. In all animals which have lungs and are viviparous, either 

 internally or externally, the lung has more blood than all the 



