VEETEBRATA. 17 



in the skull to the cranial cavity, the spinal cord, 

 which it contains, merges into the brain. In addition 

 to the cavity containing the central nervous system, 

 and placed on the upper side (= dorsal side) of the 

 animal, a cavity, the body-cavity, is found in the 

 under side ( = ventral side). It contains for the most 

 part the organs of respiration, circulation, digestion, 

 and excretion (Fig. 1), and in Mammals is divided by 

 the diaphragm into thoracic and abdominal cavities. 

 In all the other subdivisions of the animal kingdom 

 the central nervous system is situated in the same 

 cavity as the above-mentioned organs. 



Various bones are connected with the vertebral 

 column, and they serve for the attachment of muscles. 

 The bones collectively constitute the skeleton, which 

 is one of the most distinctive features of a Vertebrate. 



The animals of this sub-kingdom never have more 

 than four limbs, and their blood is red, while that of 

 most other animal groups is colourless. 



The structure of the heart in the various Verte- 

 brates must also be noticed. In no Vertebrate is this 

 organ so simple in structure as in the scheme given 

 in Fig. 6; such an arrangement, moreover, would 

 invol-s^e great difficulties. One great difficulty would 

 be that while the blood was leaving the heart at a 

 (Fig. 6), no fresh blood could enter, so that the blood 

 in the veins would stand still. Even in the lowest 

 Vertebrates (the Fishes) this difficulty is obviated, for 

 where the main vein (or veins) opens into the heart 

 an enlargement of this vein is found, where the blood 

 can collect as long as the heart continues to contract. 

 This expansion is also reckoned as part of the heart, 

 and named the auricle (Fig. 8, V.K.), while the heart 

 proper is termed the ventricle (K.). It is also easy 

 to see that there must be a tolerably wide opening 

 between the two chambers, so that as soon as the 

 ventricle becomes flaccid the auricle can force blood 

 into it. But there being such a wide aperture 



c 



