6io 



STRUCTURE OF VERTEBRATES 





■Q.aete 



•ni,.<k 



Xt^ 



Fig, 470. 



vertebrate brain is very similar to that of the dogfish (Fig. 254). 

 In the other types which we have studied (Figs. 303, 329 and 360) 

 there are variations in the proportions of the parts, but no real 

 differences of plan. In the birds, and even more in the mammals, 

 the cerebral hemispheres are enormously expanded so that they 

 stretch backwards and cover most of the rest of the brain. In 

 both groups it is the grey matter which is increased, but whereas 



in birds it is the ventral part, or 

 basal nucleus ( = corpus striatum), 

 in mammals it is the dorsal part 

 or pallium, especially that part of 

 it called the neopallium. Attempts 

 to connect these differences with 

 differences of behaviour, on the 

 assumption that birds are dom- 

 inated by instinct and mammals 

 by learning, do not, however, fit 

 the facts. Birds do show much 

 more complex instincts than most 

 mammals, but they also, as a 

 class, show a capacity for learning 

 at least comparable with that of 

 all mammals except the primates, 

 and there is no reason why nerve 

 connections to make this possible 

 should not be developed in the 

 corpus striatum. The neopallium 

 of the primates, and especially of 

 man, has, however, a complication 

 reached in no birds, and on this, in 

 a material sense, depends man's 

 ability to reason. 

 The nerves which make up the peripheral nervous system 

 develop as a continuous series down the body, but it is necessary 

 to distinguish the cranial nerves, which originate within the 

 skull, from the spinal nerves which are outside it. Both types 

 have a segmental pattern impressed on them by the mesoblast 

 which they supply, and the effect on the spinal nerves is 

 strengthened by the fact that they have to emerge between the 

 vertebrae. The segmentation of the cranial nerves has been 

 discussed on pp. 569-71. In the lampreys the dorsal roots. 



-The brain and its nerves 

 in a cod. 



c.str., Corpora striata, two large masses or 

 ganglia of grey matter which lie at the 

 base of the cerebrum of Vertebrata and are 

 here exposed because the roof of the 

 cerebrum, which in the cod is thin and 

 non-nervous, has been torn away ; c6., 

 cerebellum ; l.i., right lobus inferior ; m.o., 

 medulla oblongata ; olf.st., stalk of 

 olfactory lobe; op.l., optic lobes; II.-V., 

 VII. -X., cranial nerves; V.md., V.mx., 

 F.o/)., mandibular, maxillary, and ophthal- 

 mic branches of the fifth nerve ; Vll.hd., 

 Vll.hm., Vll.m., VI I. op., VII.p.sp., 

 Vll.pal.b., hyoidean, hyomandibular, 

 mandibular, palatobuccal, and ophthalmic 

 branches of seventh nerve ; X.c, cutaneous 

 branch of the vagus nerve, from which the 

 lateral hne is innervated. The cutaneous 

 branch of the fifth nerve is not shown. 



