276 ANIMAL PHYSIOLOGY. [LESSON 86. 



their internal ear, and their acute sense of hearing, especially in noc- 

 turnal birds. 



The spinal nerves are chiefly cervical (cervix, the neck), and sa- 

 cral (sacrum, the lower part of the back), from the number of ver- 

 tebrae composing these parts of the column. 



LESSON LXXXYI. 



NEKVOUS SYSTEM IN MAMMALIA AND MAN. 



1241. It has been shown that the nervous system in the lowest 

 animals possessing such a structure, exists in the form of a single 

 chord, presenting a lengthened axis, and in the Entozoa only one 

 ganglion is found. 



1242. To this succeeds a double chord, and with this single or 

 double chord, ganglia, or knots of nervous matter, varying alike in 

 situation and number, are associated. 



1243. Thus, in the lower Articulata, we find a ganglion, or little 

 brain, or nervous mass of reinforcement, placed in the centre of each 

 segment of the body, which are motor in their function, and supply 

 nerves only to the system of muscles found in the segments respec- 

 tively. The nerves of distribution are uniform in number, and dis- 

 tributed to similar structures in each instance the only exception 

 being in relation to the supra-sesophageal ganglion, or brain, the nerves 

 proceeding from which, with the exception of the sympathetic nerves, 

 are distributed solely to the organs of special sense. 



1244. In the Leech we found only eyes to be supplied, and thus 

 the brain distributed only optic nerves, and may be considered as 

 simply an optic ganglion. In this animal the lower portion of the 

 brain, infra-oesophageal ganglion, greatly exceeds the superior portion 

 in size the muscles to be supplied with nerves being much more 

 abundant than the organs of special sense. 



1245. In proportion as organs of sense are increased, so will the 

 brain be found to increase in size, and the greater will be the num- 

 ber of nerves distributed from its superior portion ; compared with 

 the Leech, a Caterpillar claims a much higher organization, and 

 hence the superior development of its brain, and the vast assemblage 

 of nerves in connection with it. In the Cirripeds, no organs of 

 special sense were found, neither is a superior portion of the brain de- 

 veloped, but the ganglions connected with the double chord, are mo- 



