BLOOD-SUPPLY OF BRAIN 905 



brachycephalic heads. The angle formed between the line of the central fissure and the mid- 

 sagittal plane, which averages about 68° in the adult, is more acute in dolichocephalic heads, 

 and the external part of the parieto-occipital fissure is further forward in the child, and possibly 

 in the female, than it is in the adult male. 



The position of the posterior horizontal limb of the lateral fissure varies even in the adult. 

 Its posterior part is always under cover of the parietal bone, and it terminates either in front of 

 or inferior to the parietal eminence, but the anterior part may be above, parallel with, or inferior 

 to the squamo-parietal suture. In the adult the anterior part of the fissure runs upward and 

 backward from the posterior end of the spheno-parietal suture along the anterior part of the 

 squamo-parietal suture to its highest point; thence it continues in the same direction beneath the 

 parietal bone toward the lambda, terminating either in front of or below the parietal eminence. 

 In the child, however, the fissure is considerably above the hne of the squamo-parietal suture 

 (fig. 712), which it gradually approaches, attaining its adult position about the ninth year. 

 This change of position, which occurs during the first nine years, is due partly to the ascent of 

 the sutural Hne and partly to the descent of the fissure on the surface of the brain. 



The frontal bone always covers the superior, middle, and inferior frontal gyri, except their 

 posterior extremities, which are beneath the parietal bone (fig. 711). The ascending limb (ramus 

 anterior ascendens) of the lateral fissure, which cuts into the posterior part of the inferior frontal 

 gyrus, runs parallel with and under cover of the lower part of the coronal suture, or immediately 

 in front of it, and the anterior horizontal limb is parallel with and beneath the upper margin of 

 the great wing of the sphenoid. 



The parietal bone is in relation with the convex surfaces of four lobes of the brain. Speaking 

 very generally, it may be said that the anterior third covers the posterior part of the frontal 

 lobe, including the anterior central gyrus, and the posterior ends of the superior, middle, and in- 

 ferior frontal gyri and the precentral sulcus. The posterior two-thirds of the bone are superficial 

 to the perietal lobe, the posterior part of the temporal lobe, the anterior part of the occipital lobe, 

 the posterior part of the horizontal limb of the lateral fissure, the superior and inferior parts of 

 the post-central sulcus, the interparietal sulcus, the posterior sections of the superior and middle 

 temporal sulci, and the external part of the parieto-occipital fissure. The central sulcus is 

 beneath the parietal bone at the junction of its middle and anterior thirds (fig. 711). 



In the adult, the upper end of the central sulcus is situated at about 55 per cent, of the whole 

 length of the naso-inionic line posterior to the nasion. It is about 4 or 5 cm. from the coronal 

 suture. The inferior end of the sulcus, which extends to near the posterior horizontal limb 

 of the lateral fissure, lies beneath the point of intersection of the auriculo-bregmatic line with 

 a line drawn from the stephanion (the point where the temporal ridge cuts the coronal suture) 

 to the asterion. This point is about 46 per cent, of the horizontal arc measured from the 

 glabella to the inion. 



The superior end of the parieto-occipital fissure usually lies about 5 mm. in front of the 

 lambda, and the course of the fissure may be indicated by a line drawn from 5 mm. in front of 

 the lambda to a point immediately above the asterion, and, as the latter point corresponds with 

 the pre-occipital notch on the infero-lateral border of the hemisphere, the line in question wiU 

 indicate the adjacent margins of the parietal, temporal, and occipital lobes. 



The occipital bone is in close relation with the cerebellum, as already pointed out, but it 

 also covers the posterior part of the lateral surface of the occipital lobe of the cerebral hemisphere. 

 The great wing of the sphenoid covers the outer surface of the pole of the temporal lobe, and 

 the squamous part of the temporal bone covers the anterior parts of the superior, middle, and 

 inferior temporal gyri and the sulci which separate them. 



The Blood Supply of the Encephalon 



The double origin of the continuous arterial system of the brain given by the confluence of 

 the two vertebral arteries and the two internal carotid arteries, together with the description 

 of the general distribution of the different cerebral, mesencephalic, and cerebellar arteries into 

 which the system is divided, and the origin and course of the corresponding veins, are fully dealt 

 with in Section V. Here attention may be called briefly to the abundant and systematic 

 internal distribution of the terminal branches of the system and their intimate arrangement for 

 the actual nourishment of the nervous tissues within. 



The general plan of the blood supply for the entire encephalon may be summarised as fol- 

 lows: — (1) At their origin the different arteries are so connected, directly or indirectly, on the 

 base of the encephalon, that the blood approaching the brain by way of the vertebral and 

 internal carotid arteries is practically a common supply for all the arteries of the encephalon, 

 and a given part of it may possibly pass into any one of them. (2) In the pia mater of each 

 gross division of the encephalon the different arteries again become connected with each 

 other in a superficial, freely anastomosing plexus, continuous throughout. (3) From this 

 plexus of the surface, naturally composed in part of the trunks of the different arteries 

 themselves, arise branches which enter directly into the nervous substance and which break 

 up into twigs that are terminal; i. e., twigs that do not anastomose with each other. (4) The 

 arterial capillary system arising from the terminal twigs passes over into venous capillaries 

 which converge to form corresponding venous twigs which in their turn pass to the surface 

 and join in forming a peripheral, anastomosing venous plexus superimposed upon the similar 

 arterial plexus. (5) From this venous plexus arise the different veins of the encephalon which 

 may or may not accompany the arteries for a short distance, and which finally empty into the 

 sinuses in the cranial dura mater. These, likewise confluent, empty into the internal jugular 

 veins. The chorioid plexuses of the ventricles of the brain are modifications of the general 

 anastomosing peripheral plexuses. The chorioid plexuses of the lateral and third ventricles 

 are derived largely from branches of the chorioid arteries, which arises separately from the 

 internal carotid artery. 



