2. Preparietal Branch.— Softening of foot of medifrontal and part of precen teal convolutions. Agraphia if on the left.
3. Mediparietal Branch. — Softening of both central convolutions along the ltolandic fissure, of the anterior portion of the first parietal convolution, and of the insula. In either of the last (Nos. 2 and 3) there may be aphasia due to cutting off of subcortical tracts, paralysis of face and arm on opposite side, and paresis of opposite lower extremity. Also paralytic agraphia, if on the left. Theo retically, according to Bastian, also loss of muscular sense in the fully paralyzed parts,—though impossible to demon strate.
4 and 5. Post-parietal and Temporal Branches.—Softening of the subparietal and supratemporal convolutions, and of part of the insula. Word-blindness and more or less complete word-deafness.
"It is only on rare occasions that vas cular lesions are precisely limited to the seats of particular word-centres. They are much more frequently irregular in their distribution, or multiple, and thus give rise to confused or less typical forms of speech-defect." Post-cerebral Artery. —More or less softening of the occipital lobe, especially the cortex on its inner and under aspects, including the region of the euneus, the hippocampal gyrus, and posterior por tion of lower temporal convolutions. Hemianopsia of the opposite half of the visual field, with preservation of pupil lary reactions from both halves of the retinm.
Cerebellar Arteries. — Embolic and thrombotic softening here is more rare.
Vertebral Arteries.—Embolisms of the vertebral are more often on the left, due to the existence on that side of a marked constriction where the vessel discharges into the basilar, whereby emboli are caught at that point. Softening may not result. If, however, the block extends any distance along the artery (as is usu ally the case in thrombosis), softening in the corresponding half of the oblongata may be expected.
Basilar Artery.—Blocking of this ves sel, usually thrombotic, so long as the post-communicants are patent, only pro duces symptoms by cutting off the small terminal branches to the pons. These
are, however, important, and two types of effect are distinguished according as the block affects the upper or lower por tion. Where the focus is at the upper limits of the polls, involving cerebral erns, corpora quadrigemina, and optic tract, there may be a paralysis of the ex tremities on one side with that of the eye-muscles on the other (hemiplegia alternans superior). Where, however, this affects the pons at the facial-nerve exit, there may be paralysis of extremi ties on one side with that of the facial on the opposite side (hemiplegia alter nans inferior).
Embolism.
Definition. — Embolism of the brain, like that in other parts of the body, is the blocking of an artery by a plug or material sufficiently solid to stop its blood-current. It plays a more impor tant part here because: 1. The arteries are, to a greater extent than in most other parts, terminal vessels. 2. The special functions of any destroyed part of the brain cannot be compensated; as e.g., in the lungs or spleen, where all portions act practically alike.
Varieties.— Transient Embolism.—In this form the occluding substance breaks up or is floated along to a place where the collaterals suffice, and this happens be fore death of the threatened tissues. It is believed to explain occasional tran sient seizures experienced by embolic subjects.
Simple Embolism. — The ordinary form, where the floating mass lodges in some artery and cuts off the whole cur rent immediately.
Septic Embolism—Where the embolic plug carries some infecting agent. Inas much as an ulcerative endocarditis may be due to the invasion of the ordinary pus-organisms (streptococci and staphy lococci), gonococcus, tubercle bacilli, or even certain other micro-organisms, it follows that a plug carried to the brain may be the transporter of infection like that of its source. In such cases the re action about the point of lodgment or in the involved area will bear some rela tion to the virulence of the underlying germ.