672 Dynamic Theory. 



CHAPTER LVIII. 



COMPARATIVE ANATOMY OF THE BRAIN. 



The nervous system of the amphioxus is the most simple possessed 

 by any vertebrate. ( See fig. 63. ) The only sense organs this little an- 

 imal has are a depression surrounded by cilia, near the front end of the 

 animal, supposed to be an olfactory organ, and a pigment spot a little 

 further back, supposed to be an ocellus, or the rudimentary beginning 

 of an eye. Nerve connections to these from fehe spinal cord are rather 

 inferred than proved. The medulla spinalis, or spinal cord, extends 

 from the head to the tail, as in other fishes, and gives off dorsal and 

 ventral branches at regular intervals, corresponding in number to the 

 muscular segments. Two pairs, however, appear to have already re- 

 ceived some degree of specialization. These are the first and second 

 pairs. The first pair, which are small ones, run to the parts above the 

 mouth, and may represent the seventh and eighth pair of the higher 

 vertebrates, of course not yet specialized to the uses to which they are 

 afterward put. The next pair are supposed to represent the trigeminum, 

 or fifth pair, and the par vagum or pneumogastric ( tenth pair ). This 

 nerve, after leaving the spinal cord, sends out a branch upward and 

 backward, parallel with the spinal cord and connecting with the seg- 

 mental nerves of the back ; while the main stem bends down and passes 

 backward, connecting with the ventral segmental nervos. No part of 

 the spinal cord of the amphioxus contains any ganglionic masses or en- 

 largements, and these great nerves must be considered as constituting 

 its principle condensing organ. 



Among the lower vertebrate brains are those of the Hags and Lam- 

 preys. In these fishes, in common with the Amphioxus, the spinal 

 cord is " flattened, opaline, ductile and elastic." In other fishes "it is 

 inelastic, opaque and cylindrical, or subdepressed. " ( Owen.) The cere- 

 bellum of the Lamprey is a mere commissure, or band of fibres, stretch- 

 ing across the upper end of the fourth ventricle. It is but little better 

 in the Sturgeon. ( See fig. 281.) But the development of the cerebellum 

 does not appear to keep pace with the general organization of the ani- 

 mal, for in the otherwise advanced Lepidosiren, the cerebellum is as ele- 

 mentary as that of the Lamprey. ( Fig. 290.) Owen shows the devel- 

 opment of that part of the brain among the fishes to be in accordance 

 with the activity of the animal. The Sharks and Saw-fish, therefore, 

 which are among the most active fishes, have a greatly advanced cere- 



