586 



Comparative Morphology of Chordates 



Vertebra 



Transverse 

 process-- 

 Tuberculum- 

 Capitulum-- 

 Centrum_ 



Vertebral 

 rib 



Sternal rib 



Sternebra 



Fig. 453. Thoracic arch. 



old animals it may become hardened by calcification (not ossification). 

 In the posterior region of the series of ribs, there are usually several 

 pairs of "false" ribs which are only indirectly connected to the ster- 

 num, the lower end of each being connected to one ahead of it. Behind 



the "false" ribs may be one or more 

 pairs having no connection with the 

 sternum — "floating" ribs. The ster- 

 num is more strongly developed than 

 in reptiles. It consists usually of a 

 longitudinal series of somewhat elon- 

 gated bones, sternebrae (Fig. 112E). 

 corresponding to the ribs in segmental 

 arrangements. A complete skeletal 

 segment of the thorax consists, there- 

 fore, of a dorsal vertebra, a pair of 

 vertebral ribs, each joined ventrally 

 to a sternal rib which attaches to a 

 median sternebra (Fig. 453). In sea 

 cows and some cetaceans, the sternum 

 is a single broad plate of bone showing no segmentation, this unusual 

 form being associated with the fact that the sternum is joined to only a 

 few of the more anterior ribs. 



The ribs develop in the connective tissue of the myoseptums. 

 Therefore the strongly developed thoracic ribs perpetuate the original 

 segmentation of the muscle of the thoracic wall (Fig. 460). Between 

 successive ribs extend intercostal muscles arranged in two thin 

 layers, external and internal intercostal. Contraction of these 

 muscles draws the ribs closer together. 



A complete rib, consisting of vertebral and sternal parts, forms a 

 semicircular arch pivoted at one end to the vertebral column and at 

 the other end to the sternum (Fig. 453). When the muscles attached 

 to the ribs are relaxed, the plane in which each costal arch lies is not 

 perpendicular to the sagittal plane but slopes sharply backward 

 (caudad) in relation to the plane determined by the two points of 

 attachment of the rib and perpendicular to the sagittal plane (Fig. 

 451). When, in breathing, the intercostal and other muscles attached 

 to the ribs contract, the effect is to swing each costal arch forward 

 (cephalad; therefore upward in man) on its pivoted ends. This in- 

 creases the diameter and capacity of the thoracic cavity and permits 

 external atmospheric pressure to force air into the lungs. Simultaneous 

 flattening of the diaphragm augments the increase in thoracic capac- 

 ity. Expiration of air, following relaxation of the muscles involved in 

 inspiration, results for the most part passively in consequence of the 



