HEAD AND NECK OF THE HORSE 173 



space (cisterna pontis) occurs where the basilar artery crosses the 

 ventral surface of the pons. Another coincides with the inter- 

 peduncular fossa (cisterna interpeduncularis), and is connected with a 

 fourth about the optic chiasma (cisterna chiasmatis). The latter, in its 

 turn, is continuous on each side with a cistern in the lower part of the 

 Sylvian fissure (cisterna fossae lateralis). 



Sheaths of the arachnoid are continued along the cerebral nerves for 

 a short distance, except in the case of the optic nerve, where the sheath 

 extends to the eyeball. 



At the attached edge of the falx cerebri, rounded and sometimes 

 pedunculated arachnoidal granulations (granulationes arachnoideales) 

 may often be found invaginating the dura mater and projecting into 

 the dorsal sagittal sinus and its lateral lacuna. They consist of out- 

 growths of the arachnoid enclosing subarachnoid tissue, and are more 

 obvious in old animals. 



The thin pia mater of the brain fpia mater encephali) follows all 

 the inequalities of its surface, sinking into the sulci of both the cerebrum 

 and the cerebellum, and forming sheaths for the small blood vessels that 

 penetrate the substance of the brain. The membrane is highly vascular 

 since the smaller branches of the chief cerebral arteries ramify in it 

 preparatory to their entry into the brain-tissue. When the membrane 

 is raised from the surface of the brain, its deep face is rough and 

 shaggy from the presence of minute ruptured vessels. 



Later it will be seen that folds of the pia mater are included in the 

 transverse fissure between the cerebrum and the cerebellum, and 

 between the cerebellum and the medulla oblongfata. 



Dissection. — Cautiously strip the arachnoid from tlie base of the 

 brain, and follow the arteries thus exposed as far as possible without 

 causing injury to the brain-tissue. 



The arteries of the brain. — The larger blood vessels that 

 supply the brain are contained within the subarachnoid space, and are 

 derived from three main sources, namely, the single basilar artery and 

 the two internal carotid arteries. 



A. hasilaris. — After the cerebro-spinal artery — a branch of the 

 occipital — has entered the vertebral canal by the intervertebral foramen 

 of the atlas (page 106), and has pierced the dura mater, it divides into 

 spinal and cerebral branches. The spinal branch (ramus spinalis) 

 unites with that of the opposite side of the body to form the ventral 

 spinal artery. In like manner the basilar artery is produced by the 

 union of the cerebral branch (ramus cerebralis) with its fellow. 



The single, median basilar artery enters the cranium by the foramen 

 magnum, and runs ventral to the medulla and pons to end, ventral to 



