766 



THE NERVOUS SYSTEM. 



of the membrane. Some of the round- and polyhedral-celled forms are 

 soft and very vascular, and are apt to involve the neighboring pia mater 

 and brain tissue, or the bones of the skull, which they may perforate. 

 They sometimes project through the opening in the skull in fungous, 

 bleeding masses. 



Psammomata are small globular tumors, often multiple and pedicu- 

 lated, growing from the inner surface of the dura mater. They are usu- 



FIG. 502. PSAMMOMA OF THE DURA MATER. 



ally composed of tissue fibrous, sarcomatous or endotheliomatous in char- 

 acter, and contain variously shaped calcareous concretions similar in 

 appearance to the so-called brain sand ' (Fig. 502). 



THE PIA MATER CEEEBEALIS. 



General Characters of the Pia Mater. 



The external surface of the brain is invested by a connective-tissue membrane 

 which covers the convolutions, dips doAvn into the sulci, and extends into the ventricles. 

 This membrane is abundantly supplied with blood-vessels, and from it numerous ves- 

 sels extend into the brain, so that any disturbance in the circulation of the blood in the 

 pia mater involves a disturbance in the circulation of the blood in the brain also. 



The connective tissue which makes up the pia mater is arranged in a series of mem- 

 branes and fibres reinforced by elastic tissue, so arranged as to form a spongy membrane 

 containing numerous cavities more or less filled with fluid. These cavities are continu- 

 ous with the peri vascular spaces which surround the vessels that pass from the pia mater 

 into the brain. The outer layers of the pia mater are the most compact, and are covered 

 on their outer surface by a continuous layer of endothelial cells. This external layer 

 of the pia mater is often described as a separate membrane called the "arachnoid," but 

 it is really only part of the pia. The deeper layers of the pia contain the blood-vessels. 

 The membranes and fibres which compose the pia mater are partly coated with cells 

 which have irregular and delicate cell bodies and large, distinct nuclei. 



Along- ihe borders of the longitudinal fissure, and, more rarely, on the under sur- 

 face of the brain, are a number of small, white, firm, irregular bodies, the Pacchionian 

 bodies. They vary in their size, their number, and in the extent of the surface of the 

 hemispheres which they cover. They may perforate the dura mater, or, more rarely, 

 the wall of the longitudinal sinus, and may produce erosions of the skull bones. They 

 are composed of fibrous tissue and may undergo fatty or calcareous degeneration. 

 We are ignorant as to their origin, and as to the causes of their variations in size and 



'For a study of psammoma consult Ernst, "Ueber Psarnmoma, " in Ziegler's Bei- 

 trage zur path. Anatomic, Bd. xi., p. 234, 1892. 



