PERIPHERAL NERVOUS SYSTEM. 151 



aii] The brains of Tertiary Mammals were in a very low stage of 

 aq development (comp. Fig. 126, A to F). Quite apart from the 

 arj relatively diminutive size of the brain, and more particularly of the 

 i hemispheres, its structure reminds us in many points of the Reptilian 

 \i brain, though these animals were probably related to the Ungulata 

 si arid Proboscidea. 



II. PERIPHERAL NERVOUS SYSTEM. 



By means of the peripheral nervous system a physiological 

 connection is established between the periphery of the body and the 

 central nervous system in both centripetal and centrifugal directions. 

 All parts of it, whether formed of nerve-cells or fibres, appear to 

 arise as bands or outgrowths of the central nervous system, and 

 are consequently derivatives of the epi blast. Thus the whole 

 nervous system represents a single organ morphologically as well as 

 physiologically, and the connection of the nerves with their periphe- 

 ral end-organs is to be looked upon, at least as far as we know at 

 present, as a secondary one. 



Two principal groups of peripheral nerves may be distinguished, 

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

 cord and brain respectively. The first are the more primitive and 

 simple structures, and they all show a similar arrangement along both 

 dorsal and ventral sides 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. 



This regular arrangement can no longer be plainly recognised 

 in all the cerebral nerves. Their condition in the early embryo, 

 however, shows that they have a similar origin to the spinal nerves : 

 both groups arise from two continuous longitudinal ridges of cells 

 lying along the dorso-lateral regions of the medullary cord, which 

 become later differentiated into segmentally-arranged ganglia, the 

 intersegmental portions undergoing no further development. 



Nerve-fibres (processes of the multipolar nerve-cells, consisting 

 of axis-fibres) now grow out from the dorsal region of the spinal 

 cord into this chain of ganglia, pass through them, and appear 

 again on the other side. The dorsal nerve-roots thus arise 

 secondarily, that is, after the conversion of the neural ridge 

 into a chain of ganglia, while the ventral roots are developed 

 independently from the medullary cord, and appear to be formed 

 later than the dorsal. 



We must thus bear in mind that each dorsal or sensory nerve, 

 whether it belongs to the brain or to the spinal cord, has originally a 

 ganglionin connection with it, while in the ventral nerves a 

 ganglion is wanting. 



On the distal side of each ganglion, both nerve-roots become 

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



