94 Waiter Shipley: 



supporting the weitrht of the })ody above ground ; and conversely, 

 the type is less tixed when the thorax is not made rigid by such 

 impulses. In the quadrupedal mammals, the transmission of the 

 wedght of the body from the sides of the ribs through the great 

 serrated muscles to the scapulae, and thence through the limbs to the 

 ground, narrows the cephalic end of the thorax by lateral pressure ; 

 this narrowing of the thorax, which is splendidly shown in the skele- 

 tons of the horse and giraffe, drives the lungs towards the loins and 

 keeps the apices of the lungs liehind the anterior liorder of the first 

 ribs. 



The apices of the lungs al)ove the first ribs, the normal anatdmical 

 position in man, constitutes a divergence from the normal mammalian 

 position of lung. Ribs unaffected by pressure are curved, probably 

 because betAveen cui-^^ed ribs the maximum amount of lung may 

 collect. Be that as it may, the fact stands that the first ribs are the 

 most curved ribs in the human body, and that the first ribs of the 

 horse are the straightest in its body. The first ril)s of men are prac- 

 tically never fractured, because no strain is thrown on them ; the 

 first ribs of the horse are frequently fractured ]>3' the strain thrown 

 on them by the weight of the horse's body, and also by the load 

 superimposed by man. The bearing of these facts on cervical ribs is 

 that straight first ribs prevent the hmg passing into the neofc, 

 whereas curved first ribs permit the passage of the apices of the 

 lungs into the neck, and when lungs gain the mammalian neck, ribs 

 develop over them as the occupation of the neck becomes more 

 extensive. 



Marsupials. — All these animals, including the extinct Dipi-otndon 

 ausf7'alis, are true to mammalian neck type. The kangaroos have 

 acquired a mode of progression that has led tn atrophy, from disuse 

 of the pectoral limbs and upper thorax. Owing to the inclination of 

 the body being submitted to great changes, a good degree of neck 

 curvature exists ; mostly the body is tilted downwards and forwards, 

 and it is erect for only brief periods ; thus the lungs occupy the posi- 

 tion that is normal to cpuidrupedal mammals, and they arc not 

 induced to migrate, as they do in man, towards the neck. 



The lungs of the kimgaroo do not encroach upon the neck, there- 

 fore cervit'al ril)s do not occur in these animals. 



Xenarthra. The neck structures of the animals of this order afford 

 an interesting and difficult study, interesting because of the variation 

 of type that occurs in the sloths, difficult because abundant material 

 for comparison is hard to get. 



The extinct M f(i<itlitriiiin was true to tyi)c, and tliercfore it is 

 ])robal)le that all tlu^ animals l)clonging to this order wt-re originally 

 of ordinary mainnialian tvpe. 



