202 



with the sides of the cerebellum, there is (in higher Craniates) 

 a transverse band, the pons Varolii. The bulb contains 

 centres which control the respiratory movements, the action 

 of the heart, deglutition, and other functions. 



The higher types of brain show marked flexure and 

 overlapping. The brain and spinal cord are closely invested 

 by a delicate vascular membrane, the pia mater, which is 

 thickened here and there as a very vascular choroid plexus 

 (e.g., of lateral ventricles). Outside the pia there is (in 

 higher Craniates) an arachnoid membrane. Outermost is 

 the tough dura mater, which lines the cranium and en- 

 sheathes the spinal cord. The tissues of the brain and spinal 

 cord consist of nerve cells (the white and grey matter) and 

 supporting neuroglia cells. In the brain the grey matter is 

 mostly superficial, and it forms the cortex ; but in the 

 spinal cord it is the interior tissue around the slight central 

 canal. 



Ten or twelve pairs of cranial or brain nerves are distri- 

 buted throughout the head and body ; and paired spinal 

 nerves of the cord, which usually pass out between adjacent 

 vertebrae, supply the muscles and skin. The ist cranial 

 nerve (so-called), the olfactory, is a drawn-out connection 

 between brain and olfactory organ, the 2nd or optic is 

 formed as explained on page 203-4, the 3rd, 4th and 6th 

 probably arise as outgrowths from the neural tube ; for 

 the origin of the others see page 201. Each spinal nerve has 

 two roots, a dorsal (single) and a ventral (often multiple). 

 Cf. Amphioxus, page 188. The dorsal roots have each a 

 ganglion, and are as a rule wholly afferent or sensory 

 (i.e., receiving and transmitting impressions) ; the ventral 

 roots are efferent or motor (i.e., transmitting impulses to 

 muscles). For origin of dorsal roots seo preceding answer ; 

 the ventral roots are outgrowths from the spinal cord ; 

 and the dorsal and ventral strands, bound together within 

 a common sheath of connective tissue, form the trunk 

 of the spinal nerve. 



2. The sympathetic nervous system is closely associated 

 with the spinal nerves, and consists of two longitudinal 

 ganglionated cords, one at each side of the vertebral column 

 and connected anteriorly with the cranial nerves. The 

 sympathetic ganglia are derived from the spinal and from 

 certain cranial nerves, and remain connected with them 

 by short rami communicantes, and give off nerves to the 

 alimentary canal, heart, and blood-vessels. 



