SEC. IV ANATOMY OF BIRDS 261 



verse commissure of the cerebellum, nor any corim^ callomm — that 

 great Avhite commissure of the cerebral hemispheres which is 

 characteristic of all l)ut the lowest mammals. 



The Spinal Cord, or medulla spinalis ("spinal marrow") is the 

 main nerve-axis of the body, running in the series of neural arches 

 of the vertebra? from head to tail ; it directly continues the medulla 

 oblongata. It retains its primitively tubular character in part at 

 least, and consists as usual of white matter enclosing gray matter. 

 The cord is fissured into lateral columns, as these are also to some 

 extent into anterior and posterior tracts. The latter diverge in 

 ascending the medulla oblongata, to throw the central tube into the 

 cavity of the fourth ventricle ; and especially in the sacral region, 

 where a sort of ventricle, known as the avian sinus rhomboidali^, is 

 similarly formed. The calibre of the cord increases at the root of 

 the neck, where large nerves are to be given off from the brachial 

 plexus to the wings, and again in the sacral region, with the same 

 reference to nerve-supply of the legs ; after which the cord con- 

 tinues to the end of the spinal canal as a terminal thread. 



The Cranial Nerves are twelve pairs, as in mammals, the 

 highest vertebrate number. 1, the olfactory nei've of special sense 

 (smell) ; origin from rhinencephalon ; exit from cranial ca\aty by 

 olfactory foramen, high up in orbital cavity ; conducted along a 

 groove to final escape between perpendicular and lateral plates of 

 ethmoid into the nasal chambers ; distributed to the investing 

 mucous membrane of the septal and turbinal bones of the nose. 

 The exit is through a sieve-like or cribrifmm plate only in Apteryx 

 and Dinornis (Owen). 2, the optic, nerve of special sense (sight) ; 

 origin from optic lobe and thalamus ; of great size, and forming a 

 chiasm (decussation) Avith its fellow ; exit by optic foramen, a large 

 hole in back of orbital cavity between centres of orbitosphenoid and 

 alisphenoid, close to or in common with its fellow. This nerve 

 forms the retina of the eye. 3, 4, 6, the oculimotor, pathetv:, ab- 

 ducent, collectively the motor nerves of the eye, supplying the 

 muscles moving the eyeball ; 3, to all these muscles excepting 

 superior oblique, and external rectus ; origin from crura cerebri, base 

 of mesencephalon ; 4, to the superior oblique, origin behind optic 

 lobes, upper surface of metencephalon ; 6, to external rectus (also 

 to muscles of the third eyelid in birds) ; origin between met- and 

 myel-encephalon, base of brain ; 3, 4, 6, exits from cranial into 

 orbital cavity by several small, not constant, foramina near optic 

 foramen ; or by this foramen sometimes all the nerves which enter 

 the orbit pass out of brain cavity through one great hole. 5, great 

 trifacial or trigeminal, sensorimotor ; feeling skin of head, moving 

 muscles of jaws ; origin (double) from myelencephalon ; leaves brain 

 from sides of metencephalon ; sensory root has Gasserian ganglion ; 



