184 THE CEREBRAL HEMISPHERES. 



frequently have an epithelioid arrangement : these spaces, like those of connective 

 tissue generally, doubtless serve for the passage of lymph. They can be injected 

 from the epidural space where this exists, and the injecting fluid can be forced along 

 them through the thickness of the dura mater into the subdural space. They can 

 also be filled by inserting the injecting cannula into the substance of the membrane. 

 Minute nervous filaments, derived from the fifth, tenth, and twelfth cranial nerves, 

 and from the sympathetic, enter the dura mater of the brain to be distributed 

 chiefly to the blood-vessels and to the bone, but partly perhaps to the membrane 

 itself. Nervous filaments have likewise been traced into the dura mater of the 

 spinal column. 



The arteries of the cranial dura mater are derived from various sources ; the 

 principal are three in number, viz., the anterior, derived from the ethmoidal 

 branch of the ophthalmic ; the middle, by far the largest, a branch of the internal 

 maxillary ; and the posterior meningeal, a branch of the vertebral. There are also 

 others derived from the ascending pharyngeal and occipital arteries. Their course 

 and relations have already been described (see Vol. II.). Veins accompany these 

 arteries, but others, mostly small ones, run independently and open into the venous 

 sinuses. One sinus-like vein which accompanies the anterior branch of the middle 

 meningeal artery, communicates superiorly with the superior longitudinal sinus, and 

 passes inferiorly either into the veins of the orbit, or into the diploic veins, or into 

 the sinus cavernosus (Merkel). Communicating with the superior longitudinal 

 sinus from its anterior end as far back as the beginning of the occipital region are 

 a number of diverticula, from 0'5 to 3 cm. long, which form a series of venous 

 lacunae (lacuna laterales of Key and Eetzius) receiving the independent meningeal 

 veins, and some veins from the diploe, and are invaginated by Pacchionian granula- 

 tions (see p. 190). These venous lacunae are not entirely confined to the region of 

 the superior sinus, but some may occur in the neighbourhood of other sinuses, 

 especially the lateral and straight sinus. 



Subdural space. The space between the dura mater and the arachnoid, which 

 was formerly, when the latter was considered to be a serous membrane, known 

 as the cavity of the arachnoid, is now more usually known under the above title. 

 It is in most parts a narrow capillary cleft, containing but very little fluid, which is 

 probably of the nature of lymph. Its contained fluid finds exit chiefly around the 

 arachnoid villi (Pacchionian granulations) into the sinuses of the dura mater, but 

 partly by way of the lymph-clefts in the sheaths of the issuing nerves, cranial and 

 spinal (Key and Retzius). In animals it has been shown (by Schwalbe) that the 

 space is also in communication with the deep lymphatic vessels and glands of the 

 neck and loin. Coloured fluids injected into the subdural space are never found to 

 pass into the subarachnoid space, the arachnoidal limiting membrane being every- 

 where a closed one. Nevertheless, coloured fluids can be made to pass from the 

 subarachnoid space through the arachnoid, villi into the prolongations of the 

 subdural space which surround those villi within the venous sinuses and lacunae, 

 and thence into the sinuses themselves. This passage may, however, take place by 

 filtration. 



THE PIA MATEB. 



The pia mater is a delicate, fibrous, and highly vascular membrane, which 

 immediately invests the brain and spinal cord. 



Upon the hemispheres of the brain it is applied to the entire cortical surface of 

 the convolutions, and dips into all the sulci, most of which thus contain a double 

 layer. From its internal surface numerous small vessels pass into the substance of 

 the brain, and hence this inner surface is very flocculent, and is named tomentum 

 cerebri. On the cerebellum a similar arrangement exists, but the membrane is finer, 



