J MORPHOLOGY OF THE VEINS. 893 
THE Limp ARTERIES. 
In all probability the vessels of both the anterior and the posterior extremities are derived 
from several somatic segmental arteries, the majority of which, however, in the course of phylo- 
genetic development, have atrophied. The upper limb is supplied in man by the lateral offset 
from the ventral branch of the seventh somatic segmental artery. It passes out into the 
extremity behind the shoulder girdle, courses through the upper arm, enters the antecubital 
fossa, and is continued through the forearm, in the early stages, as the anterior interosseous 
artery, to the deep part of the palm, where it terminates in the deep palmar arch. At a later 
period a median artery is given off from the parent stem, and it terminates in a superficial 
palmar arch; still later the radial and ulnar branches are given off. The latter grow 
rapidly, soon exceeding in size the parent stem, and they terminate in the superficial and 
deep palmar arches. The interosseous and median arteries decrease, and generally lose their 
direct connexions with the palmar arches. The posterior interosseous artery is also a 
secondary branch from the parent stem, and the digital arteries are offsets from the palmar 
arterial arches. 
The chief arteries of the lower extremities spring directly from the caudal arches, and may be 
looked upon as being essentially segmental; whether they represent the whole or only parts of 
typical somatic segmental arteries, however, is not clear. 
The arteries of the hind-limbs certainly show no very obvious indications of division into 
dorsal and ventral branches, though such indications are not entirely wanting. In their com- 
parative absence it is supposed that the dorsal branches have been either suppressed or incor- 
porated with the common stems; that similarly the ventral branches and their lateral offsets 
are indistinguishably fused, and that probably both are represented in a limb artery. 
The original stem vessel of the lower limb is the sciatic artery, which is continued down- 
wards behind the pelvic girdle into the popliteal and peroneal arteries, and so to the plantar 
arch. Subsequently the external iliac artery is given off from the caudal arch above the origin 
of the sciatic, and, passing into the limb in front of the pelvic girdle, it becomes the femoral 
artery. This vessel ultimately unites with the upper part of the popliteal artery, and after this 
communication is established the lower part of the sciatic atrophies and loses its connexion 
with the popliteal, which henceforth appears to be the direct continuation of the femoral trunk ; 
therefore, whilst the main artery of the upper limb is formed by the prolongation of the lateral 
branch of one segmental artery, the corresponding vessel of the lower extremity is developed 
from representatives of two somatic segmental arteries, the external iliac and femoral trunks 
being the representatives of one, whilst the popliteal and its continuation, the peroneal, are 
parts of another. 
The first main artery of the leg is the peroneal, which is continued into the plantar arch ; 
after a time, however, the posterior and anterior tibial branches are given off from the stem, 
over which, as a rule, they soon preponderate in size, and they terminate in the plantar arch, 
whilst the parent trunk diminishes and loses its direct connexion with the arch. 
The peroneal artery corresponds in position and development with the interosseous trunk 
and the anterior interosseous artery in the forearm. The posterior tibial apparently corre- 
sponds with the median artery; it develops in a similar way, and has similar relations to 
homologous nerves, the posterior tibial nerve representing the combined median and ulnar 
nerves of the upper extremity. 
The anterior tibial artery represents the posterior interosseous, whilst the radial and ulnar 
arteries of the upper extremity are not represented in the lower limb. 
MORPHOLOGY OF THE VEINS. 
Two dorsal longitudinal vessels, one on each side, connect the successive segmental veins 
together. They do not, however, in any part of their course, fuse together to form a single 
vessel comparable to the descending aorta. 
Of these dorsal longitudinal vessels, that on the right side greatly enlarges, and from it the 
main stem vessels which return blood from the body walls, the head and neck, and the limbs, are 
almost entirely formed. The left dorsal longitudinal vessel remains relatively small—in parts, 
indeed, it altogether disappears—and the blood conveyed to it by the corresponding segmental 
veins is transmitted across the middle line to the chief functional stem by later developed and 
superadded transverse communicating channels, which are formed between the more primitive 
longitudinal anastomoses. 
The primitive dorsal longitudinal anastomosing channels include on each side (1) the primi- 
tive jugular vein, (2) the primitive cardinal vein, and (3) the duct of Cuvier; the last-named 
vessel, however, is not so much a longitudinal anastomosis as a communicating channel between 
the longitudinal anastomoses and the heart, for it is formed by the junction of the primitive 
jugular and cardinal veins, and opens into the sinus venosus of the primitive heart. 
From these vessels, and from the transverse communications which are established between 
the primitive jugular and cardinal veins of opposite sides, the chief veins of the head and neck 
and the body are formed ; there are in addition, however, three later-formed vessels from which 
some, or portions of some, of the main stem vessels of the body are evolved. These later-formed 
vessels are the two internal jugular veins and the upper part of the inferior vena cava, whilst 
from the latter of these portions of the renal veins, the suprarenal veins and the spermatic 
ates: 
