76 COMPARATIVE ANATOMY. 



Myriapoda and Insecta. The cord presents many varieties 

 of form, but the general arrangement is similar to that in 

 other articulates. 



The nervous system of an insect developing by true meta- 

 morphosis presents a distinct form for each condition, as fol- 

 lows : The larval, a simple duplex gangliated cord ; the pupal, 

 in addition, a degree of coalescence of the ganglia ; and the 

 imagal or sexual, the highest degree of such coalescence. 



VERTEBRATA. Nervous system composed of a cerebro- 

 spinal axis with branches, and a sympathetic system. 



The cerebro-spinal axis is placed above the alimentary tract, 

 within a cartilaginous or osseous chamber (neural canal), and 

 consists of numbers of ganglia arranged for the most part in 

 pairs, with a tendency in separating to form median cavities 

 (ventricles). It is divided into a brain (encephalon) and spinal 

 cord (medulla spinals, myelon). In the spinal cord the gray 

 matter is arranged in ganglia of an indefinite number, and 

 placed within the white commissural mass. With compara- 

 tively few exceptions, it fills the neural canal, and contains 

 throughout its whole extent a narrow ventricle, which ex- 

 pands at the commencement of brain into the fourth ventri- 

 cle. Within the brain the gray matter is placed upon the 

 outer side of the commissural mass. The ganglia are of a 

 fixed number. With the exception of Amphioxus, this num- 

 ber is four, each ganglion receiving a distinct name, as fol- 

 lows: cerebellum, optic lobe, cerebrum, and olfactory lobe. 

 Of these the cerebellum is a single mass ; the others are in 

 pairs, excepting olfactory lobes of Rana (frog), cerebrum of 

 Squalus (shark), and the optic lobes of Lepidosiren (mudfish), 

 which present no tendency to divide. Pituitary and pineal 

 bodies constantly present ; the latter are "without calcareous 

 matter below man. 



Cranial nerves never of more than twelve or less than three 

 pairs. In Amphioxus these are (1) trigeminal, (2) pneumogas- 

 tric, glossopharyngeal, and spinal accessory represented in 

 one, and (3) hypoglossal. The olfactory and optic nerves 

 always arise from their respective ganglia. The olfactory 

 nerves are, properly speaking, those branches arising from 



