PERIPHERAL NERVOUS SYSTEM 177 



Traces of the saccus vasculosus and lobi inferiores still occur, even in Man, 

 in connection with the iiifimdibulum. 



The mid-brain is of smaller relative size than in other 

 Vertebrates. A transverse furrow across the solid optic lobes sub- 

 divides them into an anterior larger and a posterior smaller pair of 

 lobes (comp. p. 172). 



The division of the cerebellum into a median and two lateral 

 portions, already indicated in Reptiles, but much more plainly 

 marked in Birds, is carried to a still further extent in Mammals. 

 The median portion gives rise to the so-called superior vermis 

 while the lateral parts form the lateral lobes and flocculi (Figs. 143, 

 144). The two lateral lobes are connected by a large commissure, 

 the pons Varolii (Figs. 1 43-145) : this extends round the medulla 

 oblongata ventrally, and is more largely developed the higher 

 we pass in the Mammalian series. Other bands of nerve-fibres 

 connecting the cerebellum with other parts of the brain are 

 spoken of as anterior, middle, and posterior peduncles of the 

 cerebellum. 



The brain in Cretaceous Birds (e.g., Hesperornis) and in Tertiary 

 Mammals (e.g., Dinoceras, Triceratops) was much less highly developed,, 

 and he hemispheres relatively much smaller, than in existing forms. 



II. PERIPHERAL NERVOUS SYSTEM. 



Two principal groups of peripheral nerves may be distinguished f 

 viz., spinal and cerebral, that is, those which arise from the 

 spinal cord and brain respectively : by their means a physiological 

 connection is established between the periphery of the body and 

 the central nervous system both in centripetal and centrifugal 

 directions. The spinal nerves retain the more primitive and 

 simple relations, and all show a similar arrangement along both 

 dorsal and ventral regions of the spinal cord, so that each segment 

 of the trunk possesses a dorsal and a ventral pair. The former 

 consists of sensory, the latter of motor fibres (Fig. 147). 



Each dorsal or sensory nerve has a ganglion in connection 

 with it, while in the ventral nerves a ganglion is wanting, at any 

 rate in the adult. The ventral nerves arise as direct outgrowths 

 from the spinal cord, while the dorsal nerves first appear as 

 outgrowths from their ganglia, coming into connection with the 

 cord secondarily. The ganglia themselves are developed from a 

 neural ridge of epiblast cells lying close to the junction of the 

 medullary cord (p. 149) and outer epiblast. On the distal side 

 of each ganglion, both nerve-roots almost always become bound 

 up in a common sheath, though many facts seem to indicate that 

 in the ancestors of existing Vertebrates the dorsal and ventral 



N 



