THE HEAD AND NECK. 



121 



cranial aponeurosis. Follow this forward to the anterior belly. The anterior 

 belly was described with the face. 



The calvarium (Fig. 8 1 ) or second covering of the brain has three layers called 

 tables. They are named and distinguished as follows : (i) an outer, tough and 

 resisting ; (2) a middle table which is porous or spongy, called diploe. This is 

 very porous and contains many diploic veins that bleed rather freely during trephin- 

 ing. The characteristic of this table of some of the bones of the cranium is to 

 become absorbed. The intramural sinuses (see p. 1 13), as the frontal, ethmoid, 

 sphenoid, mastoid, may be considered as owing their formation to this process of 

 absorption. (3) The inner or vitreous table is hard and brittle. The inner surface 

 is grooved for the accommodation of the meningeal arteries (Fig. 81), the nutrient 

 arteries to the dura mater and calvarium. 



The meninges form the third or innermost of the three grand protectives of 

 the brain. This covering consists of a dura mater, an arachnoid, and a pia 



GONION 



OB EXTERNAL 

 OCCIPITAL PRO- 

 TUBERANCE 



BASION 



FIG. 81. THE SKULL IN SAGITTAL SECTION. 



mater. The arrangement they bear to each other makes two physiological 

 lymph spaces: (i) a subdural ; (2) a subarachnoid. These terms are self- 

 explanatory, and conform to the rule of subs in the introduction. 



The dura mater is adherent to the bony walls of the calvarium and base of 

 the skull. Its attachments are firmer in those localities where (i) great numbers 

 of nerves are leaving ; (2) where there are numerous foramina ; (3) where there 

 are many sutures. The base of the skull possesses these three characters, hence 

 here the dura is most intimately adhered. 



The dura mater has four double processes, called : (i) Falx cerebri ; (2) falx 

 cerebelli ; (3) tentorium cerebelli ; (4) diaphragma sellse. Each process possesses 

 (i) a free margin; (2) an attached margin; (3) sinuses for the reception of 

 venous blood from the brain. 



The diaphragma sellae is an inconsiderable dural process attached to the 

 margin of the sella turcica, or pituitary fossa, qn the superior surface of the body 

 9 



