Lizard Descending Pathways 
107 
works by Ariens Kappers et al. (1936), Nieu- 
wenhuys (1964), and Anthony (1970) ; re- 
cent experimental studies on the reptilian 
spinal cord have been reviewed by Cruce 
(1978). The Tegu lizard spinal cord, like all 
other reptilian spinal cords, extends through- 
out the length of the vertebral canal and 
shows no evidence of foreshortening. Since 
lizard vertebrae are morphologically more 
uniform than their mammalian counterparts, 
spinal roots are simply numbered consecu- 
tively from the first, in a rostro-caudal 
sequence. However, it should be noted that 
there is variability in vertebral numbers in 
different species of lizards, therefore the 
numbering system used here applies only to 
Tupinambis nigropunctatus. Brachial and 
lumbar enlargements are present; roots 6-9 
enter the brachial plexus and roots 24-28 
enter the lumbar plexus. 
The spinal cord can be divided into an 
exterior mantle of white matter, composed 
of fibers, and an interior core of gray matter, 
composed of cells. In reptiles, as in mammals, 
the gray matter can be subdivided into dorsal 
and ventral horns. In a microscopic analysis 
of the lizard spinal cord it was possible to 
delimit 10 regions of the gray matter which 
were similar to the “laminae” described by 
Rexed (1952, 1954, 1964) for the cat. There- 
fore, the regions were assigned the same num- 
bers used by Rexed in his analysis (Fig. 1). 
Since a detailed description of these laminae 
is available elsewhere (Cruce, 1978), in the 
present paper only certain features will be 
pointed out which will be useful for describ- 
ing the descending fiber pathways. The fol- 
lowing description of the laminae is based 
primarily on Nissl-stained material (cresyl 
violet, thionin) and fiber-stained material 
(Heidenhain) except where it is specifically 
mentioned that Golgi-stained material was 
used. 
As shown in Figure 1, laminae IV and V 
extend across the midline in all regions out- 
side the enlargements, and lamina VI is 
present only in the enlargements. Laminae 
VII is composed of a homogenous group of 
scattered medium-sized, lightly staining cells. 
It descends into the tip of the ventral horn 
to surround the lateral lamina IX group only 
in the enlargements (Fig. lA, C). In Golgi- 
stained material, cells in lamina VII appear 
to be either short-axon interneurons which 
connect with other cells in lamina VII and 
in lateral lamina IX (see below) or long- 
axon cells which give rise to ascending and 
propriospinal pathways. 
Lamina VIII is distinguished from lamina 
VII by its heterogeneous cell population. 
Whereas lamina VIII takes a medial position 
in the enlargements (Fig. lA, C), outside 
of these regions it displaces lamina VII so 
as to fill the entire ventral horn (Fig. 1B,D). 
This is consistent with its association with 
the medial lamina IX group, continuous 
throughout the cord. In Golgi-stained ma- 
terial, cells in lamina VIII generally fall 
into two classes : interneurons which connect 
with other cells in lamina VIII and in medial 
lamina IX (see below), or commissural cells 
which connect lamina VIII and medial lamina 
IX with their contralateral counterparts 
(Cruce, 1978). 
Lamina IX is composed of large, darkly 
staining cells possessing abundant Nissl sub- 
stance and is not at all laminar in formation. 
Rather, it consists of two longitudinal col- 
umns, one (present throughout the spinal 
cord) in a medial position and the other 
(present only in the enlargements) in a 
lateral position. The medial group is pre- 
sumably composed of motoneurons inner- 
vating axial or trunk musculature and the 
lateral group of motoneurons innervating the 
limbs (Ariens Kappers et al., 1936; Nieu- 
wenhuys, 1964; Romanes, 1964; Cruce, 
1974). In Golgi-stained material, motoneu- 
rons of the two lamina IX groups present 
contrasting dendritic arborizations (Fig. 2). 
The medial group of motoneurons tends to 
elaborate its dendrites within lamina VIII 
and in a somewhat radial pattern. On the 
other hand, each lateral motoneuron has two 
prominent dendrites; one extends into the 
ventral parts of lamina VIII and medial 
lamina IX before arborizing, and the other 
extends along the dorsolateral border of 
lamina VII to send branches radially into 
the lateral funiculus and into the subpial 
