Topographical Anatomy of Cortex. Localization of Functions. (Ziehen.)
FIG. 2
Topographical Anatomy of Inner Surface of Right Hemisphere. Localization of Functions. (Ziehen.)
OPERATIONS UPON THE CRANIUM.
The fissure of Rolando is the anatomical landmark whose position it is important to determine with reference to a number of modern surgical procedures, for around it cluster most of the motor areas or centres. It commences at the middle line about 56 per cent. of the distance backward from the glabella (root of the nose) to the inion (occipital protuberance), and, passing downward and forward, makes with the middle line an angle of 67 to 69 degrees. For most purposes it begins half an inch back of a point midway between the glabella and inion. It may be easily found by Chiene’s method, which consists in folding a square piece of paper diagonally and folding this again; after which it is three-quarters unfolded, the acute angle then representing 67¹⁄₂ degrees. If this be properly applied to the skull, one edge of its surface can be made to fall directly over the Rolandic fissure. The fissure may also be located by a simple instrument known as the cyrtometer—a gauged metal strip having a sliding arm upon it, which, when the long strip is placed over the longitudinal sinus (i. e., the middle line of the skull), can be made to fall directly over the fissure. While neither of these methods is invariably and minutely exact, either of them is sufficiently accurate for all practical purposes.
The fissure of Sylvius may be indicated by a line drawn from a point 3 Cm. behind the external angular process to a point 2 Cm. below the most prominent part of the parietal eminence. The short and ascending limb of this fissure is of relatively small importance in this connection.
Reid’s base-line, so called, is a line drawn from the inferior margin of the orbit backward through the centre of the external auditory meatus. It is a line often alluded to in cranial topography. The colored plate (see [Plate XLIV]) will indicate with reliable accuracy the relations of the motor centres to each other and to the principal fissures and convolutions. It pertains merely to the left hemisphere of the brain, in whose third frontal convolution is placed Broca’s centre for speech, the corresponding area upon the right side having no exactly corresponding function. The centre for vision, it will be seen, is located in the cuneus, the most basal portions of the hemispheres being the seat of the special senses of taste, smell, and hearing.
Operation.
—The word trephine is at present used both as a noun and as a verb, the older term trepan being now wellnigh discarded. The instrument consists of a section of a tube, one of whose extremities is arranged with sharply cut saw teeth, the whole provided with a grip or handle, which revolves in a plane parallel to that in which the saw teeth cut. The best instrument is that arranged in a slightly conical manner, so that it may less easily burst through the skull and do harm to parts within. The trephine proper is manipulated by the hand. A variety of substitutes have resulted from applications of human ingenuity to the problem of opening the cranial bones. Some of these are operated by foot or hand power, with reduplicated mechanisms, and others by electricity. The more complicated the mechanism the more likely it is to get out of order, and there are but few of these substitutes which give anything like lasting satisfaction.