XXlJ AXIAL SKELETON 523 



broken up into mucus, new cells take their place, being budded off 

 from the underlying Malpighian layer just as are the horny cells. 

 These pouches are known as dermal glands. 



The skull and brains are very characteristic, recalling in many 

 points those of the Dipnoi. The axis of the brain appears straight, 

 as in fishes ; in higher Vertebrates this axis is more or less folded. 

 In contrast, however, with fishes, the cerebral hemispheres of the 

 fore-brain are relatively large, and, as in Dipnoi, have a roof 

 consisting of nervous matter, whereas the cerebellum, usually so 

 large in fishes, is reduced to a mere band (Fig. 263). 



The skull always articulates by two pegs the occipital con- 

 dyles with the first vertebra (Fig. 255). It is remarkable for its 

 extremely flattened shape; the jaws are widely bent outwards so 

 that the large eyes in no way compress the cranium, which is 

 thus evenly cylindrical. Both membrane and cartilage bones are 

 present, but the ossification is by no means complete. The exact 

 arrangement of the bones will be given when a type is studied. 



The vertebrae are either procoelous (Gr. irpo, in front; KoiAos, 

 hollow), or opisthocoelous (Gr. oVio-0o-, behind), that is to say either 

 concave in front and convex behind, or vice versa, and the arrange- 

 ment may differ in allied genera, while amphicoelous. vertebrae also 

 occur. 



The vertebrae articulate with one another, not only by the 

 centra but also by facets called zygapophyses (Gr. vyoV, a yoke), 

 on the sides of the neural arches. The anterior facets, prezygapo- 

 physes, look upwards and are covered by the posterior facets or 

 postzygapophyses of the vertebra in front, which look downwards. 



The circulatory system closely resembles that of the Dipnoi. 

 The atrium is divided into two auricles, and the blood from the 

 lungs returns direct to the left auricle by the pulmonary veins. A 

 median vein, the inferior cava, returns the blood from the kidneys 

 directly into the sinus venosus, receiving in its course the hepatic 

 vein. The anterior portions of the posterior cardinals are much 

 reduced in size and may be altogether absent. 



The lungs open by a common stem, the laryngeal chamber, into 

 the throat. The opening is called the glottis, and its sides are 

 stiffened with cartilage. 



The kidneys and reproductive organs show essentially the same 

 arrangement as in the Elasmobranchs, the kidney being divided into 

 a sexual part connected with the testis and a posterior non-sexual 

 part. There is one opening for all ejecta, the cloaca. 



