98 _ The Cephalic Veins and Sinuses of Reptiles 
undoubtedly, been formed in the same way as the sinus orbitalis, the 
amount of the expansion being determined, more or less, by the nature 
of the tissues which surround the vessels. 
The chief factors in this ontogenetic process are presumably the fol- 
lowing : 
1. Heredity. 
2. The muscle mechanism which obstructs the vena jugularis interna 
and elevates the venous blood-pressure in the head. 
3. The vaso-motor and cardio-accelerator mechanisms. 
The relative importance of these different factors is difficult to de- 
termine, but it may be expressed in a tentative way by the order in which 
they are named. It is probable that hereditary influences are more 
potent in the earlier stages of sinus making, while the later development 
is due more largely to the activity of the muscle and vascular mechanisms. 
Positive evidence in regard to the value of heredity as a direct cause 
of sinus formation must be looked for in the earlier embryonic stages in 
which the muscle mechanism is not functionally developed, but the 
material at my disposal is not sufficient for the determination of this 
point. The influence of heredity is not limited, however, to the direct 
formation of sinuses; it contributes, also, the conditions which make the 
ontogenetic development possible, the most important of which, perhaps, 
is a certain indifference to pressure, which serves as a protection for the 
brain and other delicate parts. 
The activity of the muscle mechanism during the later stages of em- 
bryonic life is indicated by different physiological conditions observed in 
sections of several species. For example, in an embryo of the black 
snake, Zamenis constrictor (head 4 mm. long), the m. constrictor ven 
jugularis interne is much contracted, the vein on one side of the head 
being practically closed. The sinus orbitalis is already well developed. 
Similar conditions have been observed in an embryo turtle, Aromochelys 
odoratus (head 1.5 mm. long) and in Sceloporus undulatus (head 2 mm. 
long). On the other hand, in a specimen of Lacerta agilis (head 5 mm. 
long) the constrictor muscle is relaxed and the diameter of the vein is 
not reduced. 
In all of the cases mentioned the fibers of the m. constrictor vene 
jugularis interne are more or less immature and the observed effects are 
due to precocious activity. The effect of such activity on sinus develop- 
ment is apparent. 
