CHAP. X.] THE DUEA MATEtt. 249 



the king-crab (limulus), is evidently explained by reference to the compressed 

 form of the body, and the articulation of the legs around it. 



Some additional ganglia are met with in a few animals of the homogangliate 

 group, which seem to represent rudimentary conditions of the sympathetic 

 system. These have been best observed in insects, and they are described 

 under the name of stomatogastric ganglia. They are two or more in number, 

 connected by delicate filaments with the cephalic ganglia ; and they give off 

 long nerves, which supply the digestive organs and the dorsal vessel, or heart, 

 and which, in some instances, unite with small ganglia in the abdomen to be 

 distributed on the viscera of that cavity. 



Of the Spinal Cord and Brain. The cerebro-spinal centre is en- 

 closed in certain membranes or meninyes, which are three in num- 

 ber : the dura mater, the arachnoid, and the pia mater. 



The dura mater consists of white fibrous tissue. It is thick, 

 very strong, and flexible, without being elastic. Its fibres are dis- 

 posed on different planes, but freely intermingle. At certain situ- 

 ations, it separates into two layers to form the venous canals, 

 which are called sinuses. The inner surface of the cranial cavity 

 is covered by dura mater, which adheres closely to the bones, 

 and serves as an internal periosteum to them ; and certain pro- 

 cesses of the dura mater pass into the interior of the cranial cavity, 

 dividing it into compartments, which contain certain segments of 

 the encephalon. These processes are, 1. The/a/# cerebri, which ex- 

 tends from the crista galli to the occiput along the line of the 

 sagittal suture, and forms a vertical septum between the hemispheres 

 of the brain. 2. The tentorium cerebelli, which is attached to the 

 occipital bone along the groove for the lateral sinus, and to the 

 posterior superior edge of the petrous bone. This process forms a 

 vaulted roof to the compartment which contains the cerebellum, 

 and extends between the upper surface of that organ and the 

 inferior surface of the posterior cerebral lobes. In some animals, 

 the cat, for instance, it is partially replaced by bone. 3. The/z? 

 cerebelli, a small nearly vertical process which descends from the 

 internal occipital protuberance, and occupies the notch between the 

 hemispheres of the cerebellum. 



In the spinal canal, the dura mater does not adhere to the inner 

 surface of the spinal bones as a periosteum. On the contrary, it is 

 separated from it by a soft unctuous fat mixed with very numerous 

 veins. It adheres pretty closely to the anterior common ligament 

 which intervenes between it and the bodies of the vertebrae, and 

 seems to be continued from the cranial dura mater at the foramen 

 magnum as a funnel-shaped process of that membrane adapted to 

 the shape of the spinal canal. It ends in a blunted extremity in 



