CHAP. X.] 



CIRCULATION IN THE BRAIN. 



295 



Fig, 72. 



protection, however, 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 verte- 

 brals 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 hori- 

 zontal sweep above the atlas, and after it has pierced the occipito- 

 atlantal ligament, inclining obliquely upwards and inwards ; thirdly, 

 by the breaking up of the carotids into three branches, by the 

 inclined position of the vertebrals, and by their junction into a 

 single vessel, which takes a course obliquely upwards, and after- 

 wards subdivides into smaller branches. Such arrangements most 

 effectually break the force of the two columns, and, as it were, 

 scatter it in different directions. 



A further conservative provision is found in the manner in which 

 the blood-vessels penetrate the brain. The larger arterial branches 

 run in sulci between convolutions, or at the base of the organ ; 

 smaller branches come off from them, and ramify on the pia mater, 

 breaking up into extremely fine terminal arteries, which penetrate 

 the brain ; or these lattter vessels spring di- 

 rectly 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, whenever 

 any vessel of greater size does pierce the cere- 

 bral substance, it is at a situation where the 

 fibrous matter is external, and the part 

 perforated by foramina for the transmission 

 of the vessels. Such places are the locus 

 perforatus, the interpeduncular space, &c. 

 The accompanying figure shews the manner 

 in which the terminal arterial twigs dip ver- 

 tically into the cerebral substance, and break 

 up into a solid plexus of capillaries in the 

 stratum of vesicular matter. The capillary 

 plexus of the fibrous or white matter has 

 similar characters, only its meshes are much 

 wider (fig. 72). The capillaries of the 

 cerebral substance are easily seen to possess 

 an independent diaphanous wall, with cell- 

 nuclei disposed at intervals. The smaller 



