ANATOMICAL PROBLEMS BEARING UPON EVOLUTION. 7 



all is in the cranium, where there is no clear evidence at 

 all of osseous segmentation, but instead the formation of a 

 laminar basis cranii. 



Even accepting the essential character of the process, 

 the value and importance of segmentation are diminished 

 when a comparison is made to show its effect in the con- 

 struction of a large group of animals. We are familiar 

 with the subdivision of the mammalian spinal column 

 into regions — neck, chest, loin, pelvis and tail. The 

 causes producing this differentiation are numerous, and 

 are correlated with other differences in the skeleton— site 

 of limb attachment, tail development, length of body 

 required for the reception of viscera, &c. Among mam- 

 mals there is no constancy or definition of the number of 

 vertebral segments in any region of the spine. Even in 

 the neck, where there is greatest constancy, exceptions to 

 the rule occur. We cannot explain these differences, 

 which are fundamental and essential, and are associated 

 with other deep-seated differences of structure. 



The Peripheral Nerves. — Reference has already been 

 made to the segmental character of the peripheral nerves : 

 but even in this case the metameric arrangement is not 

 carried out completely, either in the origin or the termina- 

 tion of the nerves. There is no definite segmentation of 

 the grey matter of the spinal cord ; on the contrary, the 

 nerve loots overlap, and have relations with considerable 

 tracts of cord on each side of the point of entrance. 

 Similarly in their distribution, while some motor nerves 

 have necessarily a segmental termination in segmental 

 (intercostal) muscles, as a rule the segmental arrangemeni 

 of the nerves is obliterated or confused by the formal ion 

 of plexuses and the distribution of several spinal nerves 

 to a particular muscle or area of skin. 



These nerve plexuses produce by a combination of 



