ENDOSKELETON AND VOLUNTARY MUSCLE 



63 



adult stage and can be recognized as a thin thread extending 

 through the centra of the vertebrae and as rounded masses 

 between the concave ends of the centra, so that the actual form 

 of the notochord resembles a string of beads (Fig. 37, B). In 

 vertebrates above fishes the notochord is more and more com- 

 pletely replaced by the centra of the vertebrae until in mammals 

 it vanishes completely in the adult. The notochord may be 



NT 



Fig. 37. — A, the notochord (in solid black) as seen in a longitudinal section 

 of the larval salamander in which it represents the axial skeleton. The neural 

 tube (nt) lying above consists of the brain and spinal cord. B, diagram of a 

 median view of the spinal cord (sc) and developing centra (dc) split lengthwise. 

 The notochord (in black) is partially replaced by the enveloping centra which 

 give it a beaded appearance that remains a permanent condition in fishes. C, a 

 diagrammatic side view of two completely developed vertebrae of a mammal, 

 showing the intervertebral foramina through which nerves and blood vessels pass. 

 The nerves and blood vessels are omitted in the drawing. D, vertebra from the 

 anterior side with the spinal cord shown in section in the vertebral canal, a, 

 neural arch; c, centrum; s, neural spine; sc, spinal cord; T, transverse process; 

 az, anterior zygapophysis; pz, posterior zygapophysis. 



regarded as the primitive skeletal axis of vertebrates which in 

 the higher vertebrates is replaced by the vertebral column. 



Ribs. — There are two kinds of vertebrate ribs: (1) hemal ribs, 

 occurring in fishes and some amphibians and reptiles, and (2) 

 pleural ribs, which may occur in all vertebrate groups. In the 

 frog there are no hemal ribs. Pleural ribs are represented by 

 the small projections on the ends of the transverse processes of the 

 vertebrae. In birds and mammals only pleural ribs are present. 



