482 PHYSIOLOGICAL SERIES. 



cavities of the venous sinuses, and, apparently, by allowing the 

 cerebro-spinul fluid to escape into tin- veins, ensure an adjustment 

 of the pressure in and around the cerebro-spinal axis, when 

 the flow of blood to those organs is increased. 



The surfaces of all three sheaths are covered by a pavement 

 epithelium. 



D. 729. The head of a Sea Bream (Pagellus centronotus) in 

 sagittal section. The brain occupies only the ventral parts 

 of the spaceous cranial cavity ; it is enveloped above by a 

 deep layer of fat lodged in the subdural space. 

 Preserved in 5 per cent, formol. 



D. 730. A portion of the vascular covering, or pia mater, of the 

 brain of a Goose (Anser ferus}. The internal surl'aee i- 

 smooth like the external, corresponding to the smooth 

 unconvoluted surface of the brain which it invests. The 

 vessels are injected showing their ramifications upon the 

 membrane. O. C. 1342. 



D. 731. The cranial roof of a Dog ( Canis familiaris var. 



showing two sheet-like processes of the dura mater or outer 

 covering of the brain. One the falx project- down- 

 wards in the sagittal plane, and in it- natural position lies 

 between the dorsal parts of the two hemispheres. The 

 other the tentorium is attached transversely to the hinder 

 margin of the falx; it stretches aoroflS the posterior part of 

 the cranial cavity and is inserted between the cerebrum and 

 cerebellum. 



In the Carnivora extensive ossification occur- within the 

 tentorial membrane. In this specimen part of the mem- 

 brane has been reflected to show the bony tentorium 

 beneath. O. C. 1,11 C A. 





732. A portion of the dura mater of an Klephant 



indu-Ht) showing the union of the falx and tentorium. The 

 dura is of considerable thick))'---. and can he separated into 

 three layers, of which the outer and inner are linn and 

 compact, the intermediate zone being of looser texture. 

 and containing venous sinuses. 



