PHYLUM CHORDATA 405 



Four types of vertebra are recognized; (1) cervical vertebrce in 

 the neck, (2) dorsal or thoracic vertebrce which bear ribs, (3) 

 sacral vertebrce with which the skeleton of the hind limbs are 

 united, and (4) caudal vertebrce posterior to the sacrum. The 

 ribs support the walls of the trunk and may be united with a 

 plate-like breast-bone, the sternum. Ribs that are not attached 

 to the sternum are called false ribs. 



The appendicular skeleton serves to support the appendages 

 and fasten them to the axial skeleton. The anterior appendages 

 are joined to the pectoral girdle ; the posterior appendages to the 

 pelvic girdle. The bones of these girdles and of the appendages 

 are shown in Figure 348. The appendicular skeleton of fishes 

 is usually more simple than that of the higher vertebrates. 



Muscular System. 1 — The "flesh" of the vertebrates con- 

 sists largely of muscle. Muscular tissue is capable of contraction 

 and is responsible for all the movements of an animal. The 

 muscles are attached to the bones by tendons. The body 

 muscles are called axial, those of the appendages, appendicular. 

 The muscles of the internal organs are involuntary, i.e. they 

 do not depend upon the will of the animal (see p. 74). 



Digestive System. — The organs of digestion vary considerably 

 among the vertebrates. The mouth opens into a buccal cavity 

 which is usually provided with Jaws generally bearing teeth. The 

 teeth are used to hold the food and often to masticate it. In 

 many cases a fluid from salivary glands enters the buccal cavity 

 and is there mixed with the food, making it easier to swallow and 

 digest. Following the buccal cavity is the pharynx. In lower 

 vertebrates and in the embryos of higher forms the pharynx opens 

 to the outside by gill-slits. The oesophagus leads from the pharynx 

 to the stomach. It is usually a narrow tube, but may be en- 

 larged as in birds, to form a crop for storing and softening food. 



The stomach varies in shape and structure according to the 

 kind of food to be digested in it. Its walls contain glands which 



1 A general account of the systems of organs and their functions will be found on 

 pages 76 to 70. 



