1206 HUMAN ANATOMY. 



they encroach upon the lumen of the main channel itself, within which they appear 

 as irregularly rounded projections on its lateral walls. Whatever their relation, 

 whether with the sinus or the lateral diverticula, the Pacchionian bodies never lie 

 free within the blood-space, but are always separated from the latter by the dural 

 wall. Over the summit of the elevation the dura becomes greatly attenuated, but 

 never entirely disappears, so that only a thin membrane and the subdural cleft, 

 theoretically present but practically more or less obliterated, intervene between the 

 subarachnoid spaces and the blood-stream. This partition offers little obstruction 

 to the passage of the cerebro-spinal fluid, which, unless the pressure within the 

 venous channel is higher than that within the subarachnoid space, passes from the 

 latter into the sinus and thus relieves the intracranial tension. When well developed, 

 as they often are after adolescence but never during childhood when they are 

 small and rudimentary, the Pacchionian bodies are frequently lodged in depressions 

 within the calvaria, whose inner surface is sometimes so deeply pitted that the bone 

 in places is translucent. 



THE BLOOD-VESSELS OF THE BRAIN. 



The course and distribution of the individual blood-vessels supplying and drain- 

 ing the nervous tissue of the brain have been described in the sections on the Arteries 

 (page 746) and the Veins (page 861). It remains, therefore, only to consider at 

 this place the more general relations concerning these vessels. 



The arteries supplying the brain are derived from two chief sources the inter- 

 nal carotid and the vertebral arteries. After entering the cranium these vessels and 

 their branches form the remarkable anastomotic circuit known as the circle of Willis 

 (page 760). The latter gives off, in a general way, two sets of branches, the gang- 

 lionic for the most part short vessels which soon plunge into the nervous mass to 

 supply eventually the overlying internal nuclei, the corpora striata and the optic 

 thalami and the cortical, which pursue a superficial course and are carried by the 

 pia mater to all parts of the extensive sheet of cortical gray substance, as well as to 

 the subjacent tracts of medullary white matter. 



The medulla oblongata and the pons are supplied by branches from the anterior spinal, the 

 vertebral, the basilar and the posterior cerebral arteries. . These branches gain the nervous 

 substance as two sets, the radicular and the median. The radicular branches follow the nerve- 

 roots and, just before reaching the superficial origins of the nerves, divide into peripheral and 

 central twigs, the former being distributed superficially and the latter following the root-fibres 

 to their nuclei. The median branches are numerous minute vessels which ascend within the 

 median raphe towards the floor of the fourth ventricle and assist the centrally directed twigs of 

 the radicular branches in supplying the nuclei of the nerves situated within that region. Those 

 supplying the nuclei of the hypoglossal and the bulbar portion of the spinal accessory nerves 

 are derivations from the anterior spinal arteries ; those to the nuclei of the vagus, the glosso- 

 pharyngeal and the auditory are from the vertebral as they join to form the basilar ; whilst 

 those to the nuclei of the facial, the abducent and the trigeminal are from the basilar. The 

 choroid plexus of the fourth ventricle is provided with branches from the posterior cerebellar 

 arteries. 



The cerebellum receives its supply from three arteries, the anterior and posterior in- 

 ferior and the superior, cerebellar. The general course of these vessels is approximately at 

 right angles to the direction of the fissures and folia of the hemispheres. In the mid-brain the 

 interpcduncular space is provided with branches from the basilar and the posterior cerebral arter- 

 ies ; the cerebral peduncles with those from the posterior communicating and the terminal part 

 of the basilar ; and the corpora quadrigemina with those from the posterior cerebral, additional 

 t\vii;s passing from the superior cerebellar to the inferior colliculi. 



The thalamus is supplied by branches, all end-arteries, from different sources, those for its 

 antero-median portion being from the posterior communicating, those for its antero-lateral por- 

 tion from the middle cerebral, whilst those for its remaining parts, as well as for the pineal and 

 iln- ;^< ni( ulati- bodies, are from the posterior cerebral. The last vessel also supplies the velum 

 intrrpositiim and the choroid plexus of the third ventricle. 



Tin- structures on the base of the brain, such as the corpora mammillaria, the tuber cine- 

 reum, the- infnndihulum and the pituitary tody, receive twigs from the posterior communicating 

 arti-rit-s. The optic chiasm and tract are supplied with branches from the anterior cerebral, the 

 anterior communicating, the- internal carotid, the posterior communicating and the anterior 

 choroidal arteries. 



