NEUROLOGY 289 



serous fluid. This fltiid however does not collect; it only moistens 

 the arachnoid membrane. 



The arachnoid is located between the dura mater above and the 

 pia mater below. The pia mater adheres closely to the nerve 

 tissue. 



The falx cerebri exists in fowls and in turkeys. It has the form of 

 a segment of a circle, and extends from the middle of the interval 

 of the openings for the olfactory nerves to the tentorium cerebelli. 

 The falx cerebelli is absent. The tentorium is small and is sustained 

 by a bony plate, and there are in addition two folds, one on each 

 side, that separate the hemispheres from the tubercula quadrigemina. 

 Owing to the absence of the falx cerebelli, the meninges lie close 

 together. The falx cerebri is ossified in birds. 



The Brain Structure.— The brain (Fig. 62, C and Fig. 75, A and 

 C) is made up of three principal parts: the cerebrum, the cere- 

 bellum and the meduUa oblongata. In a fowl of mediimi size the 

 brain weighs about 150 grains. 



The pons varolii is absent in birds. The crmra cerebeUi (Fig. 

 75, No. 9) are immediately connected with the corpora restiformia. 

 The lower face of the isthmus is convex posteriorly; in front, the 

 tubercula bigemina (Fig. 75, No. 4) are united to each other by a 

 comparatively large transverse cord, formed by the optic nerves 

 intercrossing in the median line. The superior face of the meduUa 

 oblongata is depressed above to constitute a fourth ventricle; in 

 front of this ventricle are the tubercula bigemina, or optic lobes. 

 These two voluminous tubercles are separated from each other above, 

 where they embrace the cerebellum, and are salient on the sides of 

 the lower face. They are hollow internally, and communicate 

 with the aqueduct of Sylvius. The thalami optici are not well 

 developed. 



A large transverse fissure divides the cerebrum from the cerebellum 

 (Fig. 75, No. 5). The optic chiasm (Fig. 75, No. 15) behind which 

 lies the hypophysis (Fig. 62, No. C, 20) covers the region of the middle 

 brain. The large transverse fissure is the dividing line between the 

 hemispheres and the optic lobes. If the hypophysis be removed 

 there wiU be observed a slit surrounded by gray matter which is 

 called the tuber cinereum et infundibulum (Fig. 75, No. 18). 

 On the pars commissuralis and the after brain there are visible the 

 roots of the fifth, the ninth, the tenth, and the twelfth pairs of 

 cranial nerves. Close beside the median furrow which extends to 



19 



