COLUMBA 131 



superficial layer of grey matter forming the roof of the hemi- 

 spheres. The cortex of the brain in birds is thin, and markedly 

 different in this respect from that of the mammals. The 

 cerebellum is also well developed as is the rule among animals 

 which rely on a sense of balance, and its surface is thrown into 

 ridges. The median portion of the cerebellum is known as 

 the vermis, on each side of which is a conical projection known 

 as the flocculus. The front of the cerebellum is in contact 

 with the hinder surface of the cerebral hemispheres, and the 

 optic lobes which form the roof of the midbrain are thrown to 

 the side. 



As in the reptiles, there are twelve pairs of cranial nerves, 

 the spinal accessory and hypoglossal being included in the 

 skull. The wing is supplied by the brachial plexus, composed 

 of nerves from the hind part of the neck and the front of the 

 thorax. The leg is supplied by a femoral nerve, and a sciatic 

 plexus and nerve, which runs through the ilio-sciatic fissure in 

 the pelvic girdle. The obturator nerve which pierces the 

 obturator foramen innervates the region of the acetabulum. 



Sense-organs. — With regard to the sense-organs, there is 

 not much advance over the conditions in the reptiles. The 

 eye is elongated from cornea to optic nerve, instead of being 

 spherical. Projecting into the posterior chamber of the eye, 

 which is occupied by the vitreous humour (see p. 23), is an 

 upstanding vascular structure. This structure, known as the 

 pecten, arises from the spot (" blind spot ") where the optic 

 nerve and artery enter the eye ; it recalls the columella Halleri 

 which is found with similar relations in the eye of Teleost 

 fish. The function of the pecten is still dubious, but its 

 vascularity suggests that it is concerned with supplying oxygen 

 to the vitreous humour and the posterior chamber generally. 



Accommodation in the eye of the bird is peculiar. The 

 junction between the cornea and sclerotic is covered on the 

 inside by a muscle which is striated (Cramp ton's muscle). 

 Contraction of this muscle results in an increase of the con- 

 vexity of the outer surface of the eye : cornea and conjunctiva, 

 which accommodates the eye for near vision. The convexity 

 of the lens is also increased by contraction of the circular 



