Page:The Zoologist, 1st series, vol 1 (1843).djvu/356

328 and sucking-disk, so that the dissection of this part of the nervous system is rendered rather more troublesome than that of any other of the ganglionic masses. The brain gives off numerous branches to the integument and sucking-disk, and ten very minute yet distinct nerves to supply the eyes or ocelli, ten of which may be seen as little black dots around the anterior and upper margin of the mouth, with a pocket lens. From the brain two cords are continued, which soon unite and form the œsophageal ganglion, from which numerous branches are given off to the muscles about the mouth, and likewise to those which are attached to the three cartilaginous jaws. From this ganglion two cords extend backwards to the tail; to the unassisted eye, from their being so closely approximated, these two cords appear as one, but by the microscope they will be rendered quite distinct. The cords are enveloped in a strong sheath, containing much black pigment, which prevents the white characteristic nervous structure from being seen. At nearly equal distances in their course backwards, a ganglionic enlargement takes place, from which a pair of nerves are sent off on each side to supply the neighbouring parts. One of these ganglionic enlargements is represented at fig. 4, magnified about 200 diameters. When the upper surface of the ganglion is in focus, you have an appearance of striæ, which are the fibres connecting the nerves of one side with those of the other; by altering the focus so as to get the next layer into view, a number of small cells will be made evident around the margin, each one having a nucleus, and more towards the centre three or four larger cells may be seen, of an elongated oval figure, also with nuclei, and with these the nervous fibres appear to be connected. Ehrenberg, in his work on the structure of nerves, has represented a ganglion of the medicinal leech; it appears to be precisely similar to that of the horse-leech, except in size. Of these ganglia I cannot make out more than nineteen, exclusive of the first or oesophageal ganglion, which is the largest: twenty-one are generally given by authors as the number occurring in the medicinal species. As a general rule, a ganglion occurs at every fifth ring of the body; but some of them are much nearer together than others, for instance, the second ganglion from the œsophageal is much nearer to the first than any of the others until we come to the last two or three, which are placed near the anal outlet; the third, fourth, fifth and sixth are the widest apart; they then get more closely approximated