74 



ANATOMY OF VERTEBRATES. 



® 



ments, giving origins respectively to the nerves of the pectoral 

 and pelvic extremities, the slightly contracted intermediate por- 

 tion being extremely short. 



In the Marsupialia the myelon usually extends to the sacrum, 



and presents both brachial and pel- 

 vic enlargements which correspond 

 with the relative size and muscu- 

 larity of the extremities to which 

 they furnish the nerves ; the latter 

 enlargement is consequently most 

 marked in the Kangaroo, fig. 39, but 

 does not exhibit the rhomboid al 

 sinus of this part in Birds. The 

 disposition of the layer of grey 

 matter enveloping the central me- 

 dullary tract in each lateral moiety 

 of the chord is shown in the 

 three situations marked i, 2, and 

 3 ; the superior expansion and com- 

 plexity of the grey matter in the 

 anterior columns of the pelvic en- 

 largement, 3, accords with the pre- 

 dominance of the locomotive over 

 the sensory functions in the long 

 and strong saltatory legs of the 

 Kangaroo. 



In the Lissencephala we have 

 again examples of the concentra- 

 tive protraction of the myelon into 

 the dorsal region, as e.g. in some 

 Cheiroptera and in the Hedgehog. 

 From the coincidence of the condition 

 of the myelon with the tegumentary 

 covering in Erinaceus and Echidna, we are led to ask, whether 

 the shortness of the solid chord, and the great length of the suc- 

 ceeding nerves within the neural canal, have any physiological 

 relation with the habit, common to both the placental and mono- 

 trematous hedgehogs, of rolling the body into a ball when torpid 

 or asleep, or when the tegumentary armour is employed in self- 

 defence. In the bat it would seem to be concomitant with the 

 reduced size and function of the pelvic limbs : but, in the Noctules 

 ( Vespertilio noctula), the myelon extends to the lumbar vertebne. 

 The anterior enlargement is the chief one in Cheiroptera, and is close 



f) 



Myelencephalon, Macropus. 



