THE HEAD AND NECK. 125 



possible to separate the two membranes whenever this is the case. At the base 

 of the brain you will see some well-defined spaces on removing the brain. 



The Pacchionian glands will be seen as little stubby hairs or even delicate 

 enlargements on the arachnoid membrane, in the neighborhood of the superior 

 longitudinal sinus. They are enlarged villi of this membrane. They often per- 

 forate the dura mater. Occasionally they will be found to perforate the bones 

 of the skull. Their use is not positively known ; but they seem to transmit 

 subarachnoid fluid into the superior longitudinal sinus. 



Of ivliat is the pia mater composed ? 



It is composed of vessels and connective tissue. 



Where is the pia mater found in practical dissection on the cadaver ? 



The pia mater is found : (i) Covering the surface of the brain ; (2) in the 

 interior of the brain, in the third and lateral ventricles, as velum interpositum 

 and choroid plexus ; (3) forming the tela choroidea inferior along the roof of 

 the fourth ventricle. This latter is perforated by three foramina, by which fluid 

 in the ventricles communicates with fluid in the subarachnoid space. (Fig. 84.) 



SUBARA CHNOID SPA CE Superior longitudinal sinus PA CCHIONTA N BODY 



CORPUS CALLOSUM 



FIG. 85. CORONAL SECTION THROUGH THE GREAT LONGITUDINAL FISSURE, SHOWING 

 THE MENINGES. (Key and Retzius.) 



Distinguish between the dura mater surrounding the brain and the dura sur- 

 rounding the spinal cord as follows : 



1. Attachment to bone, firmly and laxly periosteal function. 



2. Dural processes, for supportive purposes. 



3. Dural sinuses, for venous blood. 



4. Pacchionian bodies ; Pacchionian villi. 



These four conditions are found in the cranial dura mater ; the dura of the 

 cord has neither attachments, processes, sinuses, nor Pacchionian bodies. 



To remove a brain take the following steps : If the case is a post-mortem, 

 then it is necessary to conduct your work so as to leave the subject in a present- 

 able form. In this case make (i) an incision from ear to ear through the scalp, 

 turn the front flap down over the face, the back one down onto the neck ; (2) 

 cut through the origin of the temporal muscle, along the complete temporal 

 ridge. Turn this muscle and its heavy fascia down over the zygoma. 



To Mark the Calvarium. Make a line from one-half of an inch above the 

 supraorbital arch on each side, around the head, passing through the external 

 occipital protuberance. Caution : Avoid getting too low in front, lest your incision 

 pass under the roof of the orbit. If you fail to bring your incision to the level 



