DURA MATER. 577 



being lined by the parietal layer of the arachnoid. The dura mater is, there- 

 fore a fibro-serous membrane, composed of an external fibrous lamella, and an 

 internal serous layer. It sends numerous processes inwards, into the cavity of 

 the skull, for the support and protection of the different parts of the brain; and 

 is prolonged to the outer surface of the skull, through the various foramina 

 which exist at the base, and thus becomes continuous with the pericranium ; its 

 fibrous layer forms sheaths for the nerves which pass through these apertures. 

 At the base of the skull, it sends a fibrous prolongation into the foramen caecum ; 

 it lines the olfactory groove, and sends a series of tubular prolongations round 

 the filaments of the olfactory nerves as they pass through the cribriform plate; 

 a prolongation is also continued through the sphenoidal fissure into the orbit, 

 and another is continued into the same cavity through the optic foramen, form- 

 ing a sheath for the optic nerve, which is continued as far as the eyeball. In 

 certain situations in the skull already mentioned, the fibrous layers of this 

 membrane separate, to form sinuses, for the passage of venous blood. Upon 

 the upper surface of the dura mater, in the situation of the longitudinal sinus, 

 may be seen numerous small whitish bodies, the glandule Pacchioni. 



Structure. The dura mater consists of white fibrous and elastic tissues, ar- 

 ranged in flattened laminae, which intersect one another in every direction. 



Its arteries are very numerous, but are chiefly distributed to the bones. Those 

 found in the anterior fossa, are the anterior meningeal, branches of the anterior 

 and posterior ethmoidal, and internal carotid. In the middle fossa are the 

 middle and small meningeal, branches from the internal maxillary, and a third 

 branch from the ascending pharyngeal, which enters the skull through the fora- 

 men lacerum basis cranii. In the posterior fossa, are the meningeal branch of 

 the occipital, which enters the skull through the jugular foramen ; the posterior 

 meningeal, from the vertebral ; and occasionally meningeal branches from the 

 ascending pharyngeal, which enter the skull, one at the jugular foramen, the 

 other at the anterior condyloid foramen. 



The veins, which return the blood from the dura mater, and partly from the 

 bones, anastomose with the diploic veins. These vessels terminate in the various 

 sinuses, with the exception of two which accompany the middle meningeal 

 artery, and pass out of the skull at the foramen spinosum. 



The nerves of the dura mater are, the recurrent branch of the fourth, and 

 filaments from the Casserian ganglion, from the ophthalmic nerve, and from the 

 sympathetic. 



The so-called glandule Pacchioni are numerous small whitish granulations, 

 usually collected into clusters of variable size, which are found in the following 

 situations : 1. Upon the outer surface of the dura mater, in the vicinity of the 

 superior longitudinal sinus, being received into little depressions on the inner 

 surface of the calvarium. 2. On the inner surface of the dura mater. 3. In 

 the superior longitudinal sinus. 4. On the pia mater near the margin of the 

 hemispheres. 



These bodies are not glandular in structure, but consist of a fibro-cellular 

 matrix, originally developed from the pia mater; by their growth, they produce 

 absorption or separation of the fibres of the dura mater; in a similar manner 

 they make their way into the superior longitudinal sinus, where they are covered 

 by the lining membrane. The cerebral layer of the arachnoid in the situation 

 of these growths is usually thickened and opaque, and adherent to the parietal 

 portion. 



These bodies are not found in infancy, and very rarely until the third year. 

 They are usually found after the seventh year ; and from this period they in- 

 crease in number as age advances. Occasionally they are wanting. 



Processes of the Dura Mater. The processes of the dura mater, sent inwards 

 into the cavity of the skull, are three in number, the falx cerebri, the tentorium 

 cerebelli, and the falx cerebelli. 

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