120 



THE BIOLOGY OF BIRDS 



"5CI4 



FiG.29. — Nervous system of a pigeon. 

 From a specimen, c.n., cerebral 

 hernispheres of the brain : o.i.., 

 optic lobes thrust to the side ; CB., 

 cerebellum ; SP.c, spinal cord ; 

 SP.N., spinal nerves ; sy.n., sym- 

 pathetic nervous sj'stem ; br.p., 

 brachial plexus to wing ; s.r., 

 rhomboidal sinus in the dilated 

 spinal cord (sp.c.) ; l.pl., lumbar 

 or sacral plexus of nerves uniting 

 to form the sciatic nerve (scN ) 

 going down the leg. 



composed of both sensory 

 and motor nerve-fibres, as 

 in the case of the vagus 

 which goes to heart, stomacli, 

 and many other parts, con- 

 trolling these, but also re- 

 ceiving tidings from them. 

 The spinal nerves, as already 

 indicated, are mixed nerves, 

 with sensory and motor 

 components. 



There is on each side of 

 the backbone a chain of 

 sympathetic ganglia. These 

 give off fibres to the adjacent 

 viscera and blood-vessels. 

 They are connected to one 

 another in a chain, which 

 is anteriorly linked to the 

 brain. Each is also con- 

 nected by a fine nerve- 

 thread with the ventral root 

 of a spinal nerve. 



As to the brain itself, it 

 is in great contrast to that of 

 reptiles, for it fills a spacious 

 cranial cavity. The cerebral 

 hemispheres are large, but 

 they are not convoluted as 

 in most mammals. Their 

 roof is rather thin and their 

 main mass consists of large 

 corpora striata which bulge 

 into the ventricles. The 

 cerebral hemispheres meet 

 the cerebellum dorsally and 

 throw the solid optic lobes 

 to the sides. To begin 



