618 VERTEBRATA. 



doubt the alliances which exist between the elaborate structures in 

 question, and the arches which exist under simpler conditions appended 

 to the vertebral segments of the trunk. 



(1652.) From an extended survey of the organization of the skeleton 

 throughout the vertebrate series, it is easy to perceive that, however 

 diversified in adaptation to external circumstances, there is a general 

 agreement between the various parts of the osseous framework, sufficient 

 to convince us that all have been constructed in accordance with an 

 ideal plan or archetype, from which <ea, as Plato expresses it, the 

 Creator has not deviated since the earliest appearance of the palaeozoic 

 races of VERTEBRATA up to the present period ; and this " archetype " 

 Professor Owen has illustrated by the diagram in the preceding page. 



(1653.) The nervous system of the VERTEBRATA is by far more complex 

 and elaborately organized than that of any of the four preceding divi- 

 sions of the animal world, and consists, in fact, of several distinct systems, 

 differently disposed, and appropriated to different offices. Certain largely- 

 developed ganglia, situated in the cavity of the cranium, generally con- 

 sidered by themselves on account of their disproportionate size when 

 compared with the other nervous centres, are commonly grouped toge- 

 ther under one common designation, and form what is called the brain, 

 or encephalon : these masses, however, as we shall hereafter see, preside 

 over various and widely-different functions, and with them perception, 

 volition, and intelligence are essentially connected. 



(1654.) Continued from the brain, and lodged in a canal formed by 

 the superior arches of the vertebral column, is a long chain of ganglionic 

 centres, so intimately united that they appear confused into a long me- 

 dullary cord, usually denominated the spinal marrow (medulla spinalis). 



(1655.) The spinal medulla in reality consists of two double series or 

 columns, composed of symmetrical and parallel ganglia, one pair of 

 columns, the anterior, presiding over those muscular movements which 

 are under the control of the will, while the posterior are destined to re- 

 ceive impressions derived from the exterior of the body ; these columns, 

 therefore, are denominated respectively the motor and sensitive tracts of 

 the spinal cord. 



(1656.) From the lateral aspects of the medulla spinalis are derived, 

 at intervals, symmetrical pairs of nerves, which escape from the spinal 

 canal by appropriate orifices situated between the different bones of the 

 vertebral column, and are distributed to the voluntary muscles and in- 

 tegument of the two sides of the body. 



(1657.) The spinal nerves, however, are not so simple in their com- 

 position as they were considered to be by the older anatomists : each of 

 them has, in fact, been found to arise from the spinal cord by two distinct 

 roots, one derived from the anterior, the other from the posterior column 

 of the corresponding side ; so that each nerve is evidently made up of 

 two distinct sets of filaments, one set communicating with the motor, 



