from being so mixed up with the dark pigment, they cannot be very readily made out: this is more particularly true as regards the trunk situated on the dorsal surface, the one on the ventral surface, from its being intimately connected with the nervous cord, is very readily distinguished. The dorsal vessel, which it will be necessary first to describe, extends from the head to the tail, along the middle line of the back, being very much obscured by pigment and cellular tissue. It is about 1⁄50 of an inch in diameter, and appears as a black line running down the middle of the body. It receives the blood from the lateral vessels by branches named dorso-lateral, and communicates freely with the branches of the abdominal vessel; about the posterior third of the body it also receives a large branch, which comes off from the intestines; this branch runs parallel with the main trunk, which then proceeds to the head, where it receives numerous branches from the œsophagus and parts around the mouth. In the whole of its course it receives a pair of branches at each segment of the body, which correspond in arrangement with the branches of the abdominal or ventral vessels.
The ventral vessel accompanies the nervous cord in nearly the whole of its course. It commences at the head, where it receives numerous branches from the sucking disk, and from the œsophagus and œsophageal nervous ganglion or brain; it also receives branches on each side of the body, at those parts where the nervous chord forms a ganglion; these branches accompany the nerves coming off from the ganglia, and so close is the analogy in the arrangement between the vessels and nerves, that many authors have confounded the two, some describing it as a blood-vessel, and others as the nervous cord. Dr. Rawlins Johnston[1] appears to have been the first to notice the ventral vessel. He describes it in the medicinal leech as a pulsating vessel, and as forming expansions in its course, where it assumes the figure of a diamond. According to this view of the subject, these expansions must correspond with the nervous ganglia; he mentions in his description of the nervous system of the same animal, that the nerve has diamond-shaped expansions, which correspond with those of the blood-vessel; or in other words, that this vessel forms a sheath for the nervous cord, (fig. 2). This opinion has been followed by Brandt and many other authors, all of whom state that the nervous cord is being continually bathed in the venous blood. Some authors make allusion to the nervous cord, and have overlooked the blood-
- ↑ Treatise on the Medicinal Leech, p. 115.
