TEE NEHVOUS SYSTEM IN BIRDS. 



895 



is nothing to remark regarding those of Birds, beyond what has been said at 

 p. 158. The envelopes or meninges are three in number, and disposed as in 

 Mammals. 



"The /a/a; cerebri is found in Birds ; in the Turkey it has the form of the 

 segment of a circle, and extends from the middle of the interval of the openin^rs 

 for the olfactory nerves to the tentorium cerebelli. The f/ilx cerebelli is absent ; 

 the tentorium is small and sustained by a bony plate, and there are, in addition, 

 two particular folds, one on each side, that separate the hemispheres from the 

 tubercula (juadrigemina " (Cuvier). Owing to the absence of the falx cerebelli, 



Fig. 481. 



A B 



BRAIN OF A BIRD. A, INFERIOR FACE ; B, SUPERIOR FACE. 



NH, Medulla oblongata; Med, s\>'\u&\ cord; HH, cerebellum; MH, optic lobe; VH, cerebral 

 hemisphere; L.o/, olfactory lobe; Hyp, hyopophysis. Tr. opt, optic tract; /, olfactory nerves. 

 The cranial nerves are indicated by the corresponding numerals. 



the meninges of Birds are closer together than those of Solipeds or Man. 

 According to Leydig, the faLx cerebri is partially ossified in birds. 



Spinal Cord. — In Birds, the spinal cord is perforated by a central canal, 

 and also offers, as in Mammals, two enlargements — a cervico-dorsal and lumbar. 

 It is prolonged into the coccygeal vertebras, and thus furnishes another proof 

 against the assertion of certain naturalists, who desire to establish a relation 

 between the length of the spinal cord posteriorly, and the development of the 

 coccygeal region. The two fasciculi of the medullary axis are separated from 

 one another at the lumbar enlargement, and afterwards join in the sacral region. 

 Between them is an elliptical space — the rJiomhoidal sinus — which is filled by 

 transparent gelatinous connective substance — a kind of efflorescence of the 

 ependymis of the central canal. 



Encephalon. — In a medium-sized Fowl, the brain weighs about 2^ drams, 

 uid comprises the three portions present in Mammaha. 



