This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
This occurs in the young from embolism; it then affects the cortex, but the more common variety is caused by thrombosis in arteries which are diseased, usually in the aged. The part farthest from the source of blood supply is the most apt to suffer, therefore we find it occurring most frequently in the anterior capsuloganglionic region, just above the usual site of apoplexy (Fig. 34). The affected area will be seen to be most remote from both the cortical and basal blood supply. The perforating arteries supplying this region are in the nature of terminal branches and do not anastomose to any extent either with each other or with the branches coming from the cortex, hence their occlusion inflicts irreparable damage.
By apoplexy is meant the rupture of a blood-vessel with consequent extravasation of blood, either in or on the brain. It may occur in any portion of the brain, and either from the arteries of the base, or from the smaller arteries of the cortex. The former is the more frequent. The arteries that most often rupture are the branches of the middle cerebral which enter the anterior perforated space, especially its outer portion. One of the largest of these anterolateral arteries, as has already been mentioned, known as the lenticulostriate, has been called by Charcot the artery of cerebral hemorrhage.
The portions of the brain usually affected are the posterior portion of the lenticular nucleus, internal capsule, and surrounding parts.
Figure 35 is a medial section of the brain, giving a lateral view of the ventricles.
Figure 36 is a horizontal transverse section of the brain, opening up the ventricles. In front are seen the two lateral ventricles, separated by the septum lucidum. The cavity shown in the septum lucidum is the so-called fifth ventricle. The round body bulging into the lateral ventricle and forming its floor is the caudate nucleus portion of the corpus striatum. The third ventricle is posterior and below the lateral ventricles, and extends from the septum lucidum in front to the posterior pillars of the fornix behind. It extends from side to side as one large cavity with no median partition. Bulging into the third ventricle on each side are the (optic) thalami. They are separated from the corpora striata by some white fibres, the taenia semicircu-laris. To the inner side of the taenia semicircularis is seen the choroid plexus, which runs down anteriorly over the thalamus to the foramen of Monro, through which it enters the lateral ventricle. Two large veins, the veins of Galen, pass down near the middle line of the third ventricle to empty into the straight sinus. Posteriorly, the choroid plexus follows the descending horn of the lateral ventricles. The choroid plexus hangs from the under surface of the velum iiiterpositum, which is a fold of the pia mater entering through the transverse fissure. The veins of Galen run between the two layers of the velum interpositum.
Turning now to Fig. 37, showing a somewhat deeper transverse horizontal section, running through the corpus striatum and thalamus, it will be seen that to the outer side of the corpus striatum and thalamus is a white layer constituting the internal capsule. It divides the corpus striatum into two parts, one to its inner side, which projects into the lateral ventricle, called the caudate nucleus, and the other to its outer side, called the lenticular nucleus. To the inner side of the posterior portion of the lenticular nucleus and internal capsule is seen the thalamus. To the outer side of the lenticular nucleus one sees other white fibres called the external capsule. An apoplectic hemorrhage occurring in the lenticular nucleus or internal capsule may push forward and rupture into the lateral ventricle. It may go backward and involve the anterior portion of the thalamus and burst into the third ventricle, and if it extends outward it involves the external capsule. Sometimes, if the hemorrhage is low down, it ruptures downward through the base of the brain, showing itself, of course, at the anterior perforated space. Fig. 38 shows these structures as seen in a medial section of the brain.
Fig. 38. - Showing the degenerative and apoplectic areas of the brain and the course pursued by the motor fibres from the curtex, through the internal capsule, crura, pons, and medulla to the decussation, where they cross the median line to supply the opposite side of the body.
In Fig. 38 there is a coronal transverse section of the brain, showing the course of fibres of the internal capsule from the cortex of the brain through the corpus striatum, between the lenticular and caudate nuclei: then, forming the crus cerebri, the fibres pass through the pons and medulla to enter the spine, decussate, and pass to the extremities. This constitutes the motor pathway from the cortex to the extremities, and when it is injured in apoplexy, the extremities of the opposite side are paralyzed.