176 GENEBAL ORNITHOLOGY. 



may be given to the simple structure thus outlined, when cells, both white and gray, but 

 especially the latter, are profusely furnished, to the oniauicntation of the mind's estate with 

 race-tracks great and small, and the place t)f fornication, — fruits of the olive, and of the arbor 

 vitae. The membranes, or meninges, which hide all this from the uninitiated, are three. The 

 pia mater, or " tender mother," which innuediately invests the brain, is very vascular, and 

 furnishes the blood supply ; not only by small arteries which immediately penetrate the sub- 

 stance of the brain, but by enfolded sheets whicli enter the ventricles, and are called choroid 

 plexus. The arachnoid, or " cobweb," comes next ; a serous fluid which it secretes bathes the 

 brain, and meets concussion with its gentler fluctuation. The dura mater, or " stem mother," 

 is a dense outer membrane which enwraps and holds the whole firmly. These meninges 

 descend into the spinal column, and answer the same purpose there, maintaining the same dis- 

 position around the spinal chord. 



The Bird's Brain ciflers the following comparative characters : It is compact, having 

 nothing of the straggling apart of its elements seen in low vertebrates, and completely fills the 

 cranial cavity. Its long axis is about transverse to the axis of the spinal column. The cerebral 

 hemispheres are well developed, but do not cover the cerebellum or optic lobes ; from their 

 dome the rhinencephalon protrudes like a porte-cochere. Their surface is quite smooth (devoid 

 of the gyri and sulci of most mammalian brains) ; even the sylvian fissure is barely indicated. 

 The optic lobes are of immense size, relatively to those of most vertebrates, and relatively to 

 the rest of the encephalon ; they appear much loosened fi-om their surroundings, at the sides and 

 lower 2Mrt of the mid-brain ; they retain their ventricles, as does also the rhinencephalon. The 

 ■corpora striata are very large. The fornix is rudimentary. The cerebellum is well developed 

 and deeply sulcate, with transverse fissures, but is not divided into right and left lobes ; a 

 " fleecy " lobule on each side, the flocculus, is well defined, and received in a special recess of 

 the inner wall of the skull. Parts of the medulla oblongata notable in mammals are obscure or 

 obsolete. There is no pons varolii, or superficial transverse commissure of the cerebellum, nor 

 any corpus callosum, — that great white commissure of the cerebral hemispheres, characteristic 

 of all but the lowest mammals. 



The Spinal Chord, or medulla spinalis (" spinal marrow") is the main nerve-axis of the 

 body, running in the series of neural arches of the vertebrse from head to tail ; it directly con- 

 tinues the medulla oblongata. It retains its primitively tubular character in part at least, and 

 consists as usual of white matter enclosing gray matter. The chord 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 rhomboidalis, is similarly formed. The calibre of the chord increases at the root of the 

 neck, where large nerves are to be given oft' from the brachial plexus to the wings, and again in 

 the sacral region, with the same reference to nerve supply of the legs ; after wliich the chord 

 continues 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 nerve of special sense (smell) ; origin from rhinencephalon ; exit from cranial 

 cavity by olfactory foramen, high up in orbital cavity ; conducted along a groove to final escape 

 between peqjendicular 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 cribriform 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) with its fellow; exit by optic foramen, a large hole in back of orbital 



