MAMMALS. 595 



The duration of pregnancy, as well as the number of young that 

 commonly come into the world at each litter, is very different in 

 different genera of mammals. The diminutive size and the slight 

 development which the young of Marsupials present on leaving the 

 uterus is remarkable. They are then received in a bag or between 

 two folds of skin at the abdomen. The small bones attached to 

 the fore part of the pelvis (p. 564) serve to support this sac. To 

 the nipples placed at the inside of the sac, which are very long, 

 but which except at the time of suckling are drawn in by reversion 

 like the finger of a glove 1 , the young are now fixed and continue 

 to hang for some weeks. Afterwards, when they are more deve- 

 loped, they quit the nipple, but the sac remains a hiding-place for 

 some time, and to it the young animals resort when danger or pur- 

 suit threatens. Amongst the marsupiates which have no sac, but 

 of which the teats are placed between two folds of skin, some bear 

 their young on their back during the period of more advanced 

 development, whilst the tails of the progeny are convoluted round 

 that of the parent (DidelpMs murina, dorsigera, &c.) . 



The central nervous system of mammals corresponds to that of 

 birds in this respect, that the mass of the spinal marrow is much 

 less than that of the brain. In mammals also, as in the rest of the 

 vertebrates, a narrow canal often runs longitudinally in the middle 

 of the spinal marrow 2 ; in others, however, as in man it is present 

 in the embryonic state alone. The number of pairs of spinal 

 nerves differs in different species, as might be inferred from the 

 different number of vertebrae. Since the dura mater which covers 

 the spinal marrow or cord extends further back in the vertebral 

 canal than the cord itself, the last pairs of nerves before perforating 

 this membrane have an oblique course, and the cauda equina is 

 formed which is scarcely ever met with in the cord of the other 

 vertebrates. This arrangement is more conspicuous in proportion 

 as the cord is shorter ; the bats arid the hedgehog are distinguished 

 by an unusually short spinal marrow, in them it ends before the 

 last dorsal vertebra. The inferior cervical nerves form with the first 



earlier works compare OKEN und KIESER Beitrage zur vergl. ZooL, Anat. und Physiol. 

 des Darmkanals aus der Vesicula umbilicalis dargestelt im menschlichen Enib-yo, Mit i 

 Kupfert. Gb'ttingen, 1810, 4to. 



1 Compare J. MORGAN Linncean Transact, xvi. pp. 455 463, PL 26. 



2 See W. SEWEL Philos. Transact. 1809. 



382 



