NERVOUS CENTRES. (HUMAN ANATOMY. THE ENCEPHATON.) 



arteries are distributed to the velum interpo- 

 situm. 



The posterior cerebral arteries are the largest 

 branches of the basilar. They diverge and pass 

 upwards and backwards round the crus cerebri, 

 and reach the inferior surface of the posterior 

 lobe, anastomosing in the median fissure with 

 ramifications of the anterior cerebral, and on 

 the outside with branches of the middle cere- 

 bral arteries. Numerous small vessels pass from 

 these arteries at their origin, and penetrate the 

 interpeduncular space, and one or two are dis- 

 tributed to the velum. Shortly after its origin 

 each of these arteries receives the posterior com- 

 municating branch from the carotid. 



A remarkable freedom of anastomosis exists 

 between the arteries of the brain. This takes 

 place not only between the smaller ramifica- 

 tions, but likewise between the primary trunks. 

 The former is evident all over the surface of 

 the cerebrum and cerebellum. The latter con- 

 stitutes the well-known circle of Willis. This 

 anastomosis encloses a space, somewhat of an 

 oval figure, within which are found the optic 

 nerves, the tuber cinereum, the infundibulum, 

 the corpora mamillaria, and the interpedun- 

 cular space. The anterior communicating ar- 

 tery, between the anterior cerebral arteries, com- 

 pletes the circle in front. The lateral portion 

 of the circle is formed by the posterior com- 

 municating artery, and it is completed behind 

 by the bifurcation of the basilar into the two 

 posterior cerebral arteries. Thus, a stoppage 

 in either carotid, or in either vertebral, would 

 speedily be remedied. The coalescence of the 

 vertebrals to form the basilar affords conside- 

 rable security to the brain against an impedi- 

 ment in one vertebral ; and, should the basilar 

 be the seat of obstacle, the anastomoses of the 

 inferior cerebellar arteries with the superior 

 ones would ensure a sufficient supply of blood 

 to that organ. If either or both carotids be 

 stopped up, the posterior communicating arte- 

 ries will supply a considerable quantity of 

 blood to the intracranial portions of them ; or, 

 if one carotid be interrupted, the anterior com- 

 municating branch will be called into requisi- 

 tion to supply blood from the opposite side. 



Interruption to the circulation in both caro- 

 tids and both vertebrals is productive of a com- 

 plete cessation of cerebral action, and death 

 immediately ensues, unless the circulation can 

 be quickly restored. This was proved clearly 

 by Sir A. Cooper's experiments on rabbits. 

 The circulation may, however, be interrupted 

 in both carotids, or in both vertebrals, without 

 permanent bad effect ; or in one carotid or one 

 vertebral, provided the condition of the remain- 

 ing vessels be such as not to impede the circu- 

 lation in them. In cases where the neighbour- 

 ing anastomotic branches are not sufficient to 

 restore the circulation to a part from which it 

 has been cutoffby the obliteration of its proper 

 vessel, the cerebral substance of that region is 

 apt to experience a peculiar form of softening* 



* In the last volume of the IVTed. Chir. Trans. I 

 have relatpd a remarkable case in which white 

 softening of one hemisphere followed the plugging of 

 the common carotid on the same side by coagulum. 



VOL. III. 



or wasting, which is distinguished by the ab- 

 sence of any discoloration by the effusion of 

 blood, or of any new matter. 



The four great channels of sanguineous sup- 

 ply to the brain are continued up straight from 

 the aorta itself, or from an early stage of the 

 subclavian. The columns of blood contained in 

 them are propelled very directly towards the 

 base of the brain, through wide canals. Were 

 such columns to strike directly upon the base 

 of the brain, there can be no doubt it would 

 suffer materially. Considerable protection, how- 

 ever, is afforded to the brain ; first, by the blood 

 ascending against gravity, during at least a 

 great portion of life ; secondly, by a tortuous 

 arrangement of both carotids and vertebrals 

 before they enter the cranial cavity; the carotid 

 being curved like the letter S in and above the 

 carotid canal, and the vertebral being slightly 

 bent between the atlas and axis, then taking 

 a horizontal sweep above the atlas, and after it 

 has pierced the occipito-atlantal ligament, in- 

 clining obliquely upwards and inwards ; thirdly, 

 by the breaking up of the carotids into three 

 branches ; by the inclined position of the ver- 

 tebrals, and by their junction into a single 

 vessel, which takes a course obliquely upwards, 

 and afterwards subdivides into smaller branches. 

 Such arrangements most effectually break the 

 force of the two columns, and, as it were, scat- 

 ter it in different directions. 



A further conservative provision is found in 

 the manner in which the bloodvessels penetrate 

 the brain. The larger arterial branches run in 

 sulci between convolutions, or at the base of 

 the brain ; smaller branches come oft' from 

 them, and ramify on the pia mater, breaking 

 up into extremely fine terminal arteries, which 

 penetrate the brain ; or these latter vessels 

 spring directly from the larger branches, and 

 enter the cerebral substance. As a general 

 rule, no vessel penetrates the cortical layer of 

 the brain, which, in point of size, is more than 

 two removes from the capillaries ; and, when- 

 ever any vessel of greater size does pierce the 

 cerebral substance, it is at a place where the 

 fibrous matter is external, and that part is per- 

 forated by foramina for the transmission of the 

 vessels. Such places are the locus perforatus, 

 the interpeduncular space, &c. The capillaries 

 of the cerebral substance are easily seen to 

 possess an independent diaphanous wall, with 

 cell-nuclei disposed at intervals. The smaller 

 arteries and veins can also be admirably studied 

 in the pia mater of the brain. 



The venous blood is collected into small 

 veins, which are formed in the pia mater at 

 various parts of the surface, and in the interior 

 of the brain. The superficial veins open by 

 short trunks into veins of the dura mater, or 

 into the neighbouring sinuses ; the superior 

 longitudinal, the lateral, and the straight sinuses 

 receiving the greatest number. Those from the 

 interior form two trunks, vena mugnts Galrni, 

 which pass out from the ventricles between the 

 layers of the velum interpositum. The cere- 

 bral veins are devoid of valves. 



We remark here, that the venous blood of 

 the brain is returned to the centre of the circu- 



2 7, 



