222 FIRST LESSONS IN ZOOLOGY. 



mammals) are the presence in the cat of a circular muscle 

 called the "diaphragm" (Fig. 228, di), which divides the 

 body-cavity into two compartments, the thoracic cavity and 

 the abdominal cavity; in the former lie the heart and lungs, 

 and in the latter the stomach and intestines. In the bird 

 the aorta, or great artery given off by the heart, turns to 



FIG. 228. n, brain and spinal marrow contained within the skull and backbone, 

 which are deep black; st, breastbone; d <i, alimentary canal; s, stomach; h, 

 heart; c c, great blood-vessels; u, bladder; sy, chain of sympathetic ganglia; 

 di, diaphragm. 



the right, but in the cat to the left. And then there are 

 many other differences, in most of which the cat shows its 

 superiority both to the reptile and to the bird. 



Fig. 229 illustrates the general anatomy of the cat; the skin and 

 right half of the body-wall having been removed. The body-cavity is 

 divided into an anterior and posterior division by a transverse arched 

 partition, the diaphragm (D), composed of a thicker peripheral mus- 

 cular portion and a thinner central tendinous part. Through the lat- 

 ter pass the great blood-vessels and the oesophagus. The anterior 

 chamber is the thorax or pleural cavity, and contains the respiratory 

 organs and heart. To show these, the right lung has been removed. 

 The heart (Hf) was inclosed in the thin walled pericardia! sac, which 

 has been cut away. The great systemic veins enter from behind i.e., 

 dorsal ly; from below the vena eava inferior (V), passing up through the 

 diaphragm and uniting opposite the heart with the large vena cava su- 

 perior, from above, the two emptying into the right auricle. The 

 oesophagus (Oe) overlies the trachea (TV). The aorta arises from the 

 heart, and, curving upward and backward, runs to the left of both 

 trachea and oesophagus, as indicated by the dotted lines, and con- 



