ENCEPHALIC CIRCULATION. 



485 



lacing white iibres, the testes, in addition, having an escternal 

 white tunic. 



The brain is developed from three cerebral vesicles, the 

 cavities in which relatively represent the third and fourth 

 ventricles and the aqueduct of Sylvius ; hence these three 

 spaces and their surroundings may be said to constitute the 

 primary division of the brain, and the parts have been named 

 respectively the fore, the middle, and the hind brain. The 

 lateral ventricles and hemispheres are regarded as oflfshoots or 

 expansions of the fore- brain. 



Fig. 1£2. 



Longitudinal, vertical, and diagramatic section of a brain. Hmp., Cerebral 

 hemispheres; Py., Pituitary body; Tli.E., Thalamencephalon, or fore-brain 

 (the dark line joining Pn. and Py. represents the lamina terminalis) ; C.Q., 

 Corpora quadrigemina ; C.C., Crura cerebri ; P.V., Pons VaroUi. The origins 

 of the twelve cranial nerves are marked in order,- in roman figtires. For 

 other descriptions, see explanation of Fig. 180. 



ENCEPHALIC CIRCULATION. 



The circulation of the blood in the brain differs in a remark- 

 able degree from that of any other region, one great peculiarity 

 being that the general arrangement of the vessels in, the substance 

 of the organ is almost entirely capillary. The larger arteries all 

 ramify in the dura mater, and in the sulci between the convolu- 

 tions, and the smaller branches, from which the extremely fine 

 penetrating twigs are given off, in the pia mater. The veins 

 also are extremely small, and as a rule do not unite to form 

 trunks until they emerge from the nervous substance, while their 

 superficial branches, dispersed over the surface of the brain, enter 

 large sinuses or canals within the dura mater. These sinuses 

 are remarkable for their rigidity, which enables them to preserve 

 an almost uniform diameter. Another remarkable feature is the 

 great freedom of anastomosis which exists among the arteries, in. 

 the larger trunks, as well as in the smaller branches. 



