117 



CHAPTER XYI. 



NERA OUS SYSTEM OF AVES. 



§ 137. Myelenceplialon of Birds.— The myelon, with its nerves, 

 having led to the developement of protecting arches in the em- 

 bryo, soon ceases to be coextensive with the neural canal, shrink- 

 ing from the hind part of it, as the caudal vertebrae begin to be 

 modified, and leaving there but a filamentary trace of its original 

 condition : the neurine accumulates in the sacral region, fig. 38, s, 

 as the pelvic members grow, and the central canal there expands, 

 ib. t. The myelon becomes more slender in the dorsal region, 

 and again expands near the base of the neck, in connection with 

 the nerves of the wings, ib. u, v, iv ; then, resuming its dorsal 

 dimension, it is continued to the brain, ib. a- e. The expansions, 

 or at least the pelvic one, present a full transverse ellipse in sec- 

 tion, the rest of the chord is cylindrical; it consists of white 

 neurine with grey matter internally originating nerve-roots, and 

 lining a subcentral canal. 



The myelon is divisible into ventral and dorsal tracts accord- 

 ing to their relative position to the transverse plane of the canal ; 

 the ventral tract is actually divided by a longitudinal fissure into 

 symmetrical halves or ' columns,' the fissure extending from the 

 exterior nearly to the canal from which it is separated by a very 

 thin commissural tract unitino^ the ventral columns. The dorsal 

 ones diverge from each other in the sacrum, forming the cavity 

 above mentioned, called in Ornithotomy ^ sinus rhomboidalis,' 

 fig. 38, s, t, which is a Ventricular' dilatation of the my clonal 

 canal. The longitudinal fissure thence continued between the 

 dorsal columns becomes less conspicuous than the ventral fissure, 

 and appears to be obliterated in the neck : but the dorsal co- 

 lumns diverge in the medulla oblongata, as in the sacrum, and 

 again expose the myelonal canal, which is here called '^fourth 

 ventricle,' ib. d. 



The two expansions of the bird's myelon vary in relative size 

 according to the different developement and powers of the wings 

 and legs : the anterior or alar enlargement is greatest in Volito- 

 res, especially the Swifts and Humming-Birds : the posterior or 



