Further details follow. To put the matter shortly in modern terminology, Celsus introduced a needle through the sclera and choroid into the vitreous chamber, and depressed the lens from behind, after first rupturing its posterior capsule by vertical strokes made with the point of the instrument. As already mentioned, Galen (born A.D. 131) states that there were both in Alexandria and in Rome surgeons who confined themselves to operating upon cataract. Apparently he also described his own procedure, for some five centuries later Paulus Ægineta (_circa_ A.D. 630), in detailing his technique, gave Galen the credit for it. There is practically no difference between the method they both employed and that originally laid down by Celsus. A point of real interest in this connection is that the description of the operation given by Paulus is practically the only one extant from the pen of a Greek author, though not a few of them mention couching and advocate it. For the next landmark in the study of the subject we have to pass over three and a half centuries, till we come to the writings of Avicenna (_circa_ A.D. 980), in which we find introduced a new feature in the technique; for he mentions that the Arab surgeons used two instruments for couching--viz., a two-edged lancet with which they made a corneal incision, and a needle with which they depressed the cataract, after introducing it through the incision thus made. In this needle there was an eye near the point, through which a thread was inserted. According to Avicenna, the object of this was to help depress the lens, but it seems at least possible that the thread passed through this eye was wound round the instrument, and so served as a stop, similar to that used by the Indian coucher to-day. In any case the description is of great interest, linking as it does the Mahomedan operator of the twentieth century with his predecessor of the tenth. The famous Spanish surgeon Abu El Kasim adopted exactly the same technique for couching as that we have just described. This, as has already been suggested, is not in the least remarkable, for his name bespeaks his Arab descent.