484 VERTEBRATA. 



elements not found in the human body. If we prosecute our sur- 

 vey a little further, we shall find this portion of the skeleton offer- 

 ing the greatest possible variety as regards the presence or absence 

 of the elements above enumerated : thus in the Frog we have 

 vertebrae and sternum, but no ribs ; in the Serpent, vertebrae and 

 dorsal ribs, but no sternum or sternal ribs ; in Man the sternal 

 ribs are represented by the costal cartilages ; and thus a thorax of 

 every required description is constructed by adding or taking away, 

 expanding or contracting certain elements, all of which a typical 

 skeleton might be supposed to contain developed in a medium 

 condition. 



The nervous system of the Vertebrata is by far more complex 

 and elaborately organized than that of any of the four preceding 

 divisions of the animal world ; and consists, in fact, of several dis- 

 tinct systems differently disposed and appropriated to different 

 offices. Certain largely developed ganglia situated in the cavity 

 of the cranium, generally considered by themselves on account of 

 their disproportionate size when compared with the other nervous 

 centres, are commonly grouped together under one common de- 

 signation, 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. 



Continued from the brain, and lodged in a canal formed by the 

 superior arches of the vertebral column, is a long chain of gan- 

 gl ionic centres, so intimately united that they appear confused 

 into a long medullary cord usually denominated the spinal marrow 

 (medulla spinalis). 



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 receive impressions derived from the exterior of the 

 body : these columns, therefore, are denominated respectively the 

 motor and sensitive tracts of the spinal cord. 



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 mus- 

 cles and integument of the two sides of the body. 



The spinal nerves, however, are not so simple in their conipo- 



