7 o ELEMENTA R Y ANA TOMY. [LESS. 



In the Whalebone Whales after the first few ribs the 

 " heads " and " necks "entirely disappear, the more post-axial 

 ribs being attached by their " tubercles " only. 



The peculiar proportions of the head and tubercle of the ribs 

 in man are not universal, but seem to be special modifica- 

 tions of a more primitive type, such as exists in the Ichthyo- 

 saurus and in some Tailed-Batrachians. 

 In them the rib divides at its proxi- 

 mal end into two diverging and equal 

 processes, the upper of which (answering 

 to the tubercle of man) articulates with 

 the dorsal, or tubercular, transverse pro- 

 FIG. 79. VERTEBRA OF cess, while the lower (answering to the 

 AXOLOTL. neck and head of man's rib) articulates 



s, neural spine ; d, tuber- w [fa ^g ventral, or capitular, transverse 

 &SSSaS Process. In some other Tailed-Batra- 

 tion of nb;/, capitular chians the proximal end of the rib bears 

 SS bifu S r U cS in o g f * double facet, and articulates with a 

 rib. similarly facetted transverse process, 



which thus evidently answers to an 

 upper and a lower transverse process united into one. 



The relation borne by these articular surfaces to the neuro- 

 central suture is not constant. Sometimes, as in man, the 

 " head" articulates mainly above that suture ; sometimes, as 

 in Monotremes, altogether below it. In Ichthyosaurus 

 both surfaces are attached altogether below that junction, 

 while in Plesiosaurus the point of attachment rises as we 

 proceed backwards. 



The class of Fishes shows what abnormalities are possible 

 with respect to the modes of attachment of the ribs, as in 

 Batrachus they have a more and more dorsal origin, until, at 

 the anterior end of the body, they are actually attached to 

 the neural spines ! 



That double attachment which exists in man between the 

 proximal end of the rib and two vertebras is not universal. 

 Thus in Birds it is only the last rib, or the last but one, 

 which is attached to the point of junction of two adjacent 

 vertebral bodies. 



The greater breadth of the first rib of man is a character 

 frequently found in his class and sometimes much exaggerated, 

 as in the Great Armadillo. Often in Mammals, and in lower 

 forms generally, it is only of similar breadth to those post- 

 axial to it. 

 M The greater curvature in man of the first rib is a character 



