These commence at the foot with a series of plexuses, which may be described as forming (1) An Internal Or Intra-Osseous Venous System, and (2) An External Or Extra-Osseous Venous System.
1. The Intra-Osseous Venous System. - This is a venous system within the structure of, and occupying the semilunar sinus of the os pedis. It follows in every respect the arrangement of the arteries as before described in the same region. Efferent vessels emerge from the plantar foraminae, follow the plantar fissures, and ascend within the basilar processes of the os pedis. Here they lie under shelter of the lateral cartilages, and assist in the formation of the deep layer of the coronary plexus of the extra-osseous system.
2. The Extra-Osseous Venous System. - This may be regarded as a close-meshed network enveloping the whole of the foot. Although a continuous system, it is best described by recognising in it three distinct parts:
(a) The Solar Plexus.
(b) The Podophyllous Plexus.
(c) The Coronary Plexus.
(a) The Solar Plexus. - The veins of this plexus discharge themselves in two directions: (1) By a central canal or canals running along the bottom of the lateral lacunae of the plantar cushion to gain the deep layer of the coronary plexus. (2) By the Circumflex or Peripheral Vein of the Toe, a canal formed by ramifications from the solar and the podophyllous plexuses, and following the direction of the artery of the same name. The circumflex vein terminates by forwarding branches to concur in the formation of the superficial coronary plexus.
(b) The Podophyllous or Laminal Plexus. - The podophyllous veins anastomose below with the circumflex vein of the solar plexus, and above with the veins of the coronary plexus.
(c) The Coronary Plexus. - This proceeds from the podophyllous, the intra-osseous, and the solar networks, and consists of a central and two lateral parts.
The central portion lies between the lateral cartilages and immediately under the coronary cushion. The lateral portions are ramifications on both surfaces of the lateral cartilages. The ramifications on the lateral cartilages may be again distinguished as superficial and deep. The superficial layer is distributed over the external face of the cartilage, forming thereon a dense network, and finally converges towards the superior limit of the plexus to form ten or twelve principal branches, which again unite to form two large vessels. These vessels, by their final fusion at the lower end of the first phalanx, constitute the digital vein. The deep layer is formed, as before described, by ascending branches from the posterior parts of the podophyllous and solar plexuses, and by branches from the intra-osseous system of the pedal bone. The veins of this deep layer finally drain into the two vessels proceeding from the superficial layer, which go to the formation of the digital vein.
The Digital Veins - These arise from the network formed on the surfaces of the lateral cartilages, and ascend in front of the digital arteries to unite above the fetlock, where they form an arch between the deep flexor and the suspensory ligament. From this arch (named the Sesamoidean) proceed the Metacarpal Veins.
The Metacarpal Veins. - Three in number, they are distinguished as an Internal and an External Metacarpal, and a Deep or Interosseous Metacarpal. As we shall be concerned with these in the higher operation of neurectomy, we may give them brief mention.
The Internal Metacarpal Vein, the largest of the three, has relations with the internal metacarpal artery and the internal plantar nerve. These relations were shortly discussed under the section devoted to the arteries, to which the reader may refer.
The External Metacarpal Vein. - This ascends on the external side of the flexor tendons in company with the external plantar nerve.
The Interosseous Vein. - This is an irregular vessel running up between the suspensory ligament and the posterior face of the large metacarpal bone.