108 The Cephalic Veins and Sinuses of Reptiles 
the heart to the brain, room must be made for it by the increased exit of 
the fluids already present; for any pressure on the brain substance be- 
yond a certain limit is injurious to its welfare and activity. Some room 
may be provided by the escape of cerebro-spinal fluid from the skull. 
But within the limits of the normal cerebral circulation the characteristic 
venous sinuses especially serve to regulate.the internal pressure. ... . 
The injurious effects of too great a pressure on the brain substance are 
seen in certain maladies, where blood passing by rupture of a blood- 
vessel out of its normal channels remains effused on the surface of the 
brain or elsewhere, and by taking up the room of the proper brain sub- 
stance leads, by ‘compression,’ as it is called, to paralysis, loss of con- 
sciousness, or death.” | 
In the light of the foregoing account the head of the reptile presents 
a combination of apparently irreconcilable conditions. On the one hand 
we find intracranial sinuses designed apparently for the protection of the 
brain. On the other hand we see a mechanism for developing high blood- 
pressure, which must necessarily affect the brain as well as other sensitive 
parts. The fact that no serious consequences arise as a result of high 
blood-pressure might be explained on the assumption that the brain of 
reptiles is naturally less sensitive to pressure than is the more complex 
brain of higher forms. It is more than probable, however, that the 
brain of reptiles has acquired a certain immunity to pressure by reason 
of its long exposure to peculiar conditions. 
Whatever the explanation may be, it is evident that the brain of lizards, 
snakes, and turtles is not at all sensitive to the pressure due to the dis- 
tension of the veins of the cranial cavity. Hence, also, the intracranial 
sinuses lose their importance as a means of protecting the brain from 
the possible dangers of high pressure, for they fail at the time when the 
pressure is greatest. At such a time, they probably serve to equalize 
the blood-pressure within the cranial cavity or between the cranial cavity 
and other parts of the head. Their chief action, however, seems to be 
limited to ordinary conditions, when they regulate the intracranial blood- 
pressure, not as a means of protecting the brain, but in order to facilitate 
changes in the internal blood supply of that organ. 
VII. SUMMARY OF PART SECOND. 
1. The venous sinuses of the head of lizards, snakes, and turtles are 
associated with a mechanism which causes distension of veins and sinuses 
and thus produces intumescence and enlargement of the head. 
2. In all three orders this swell mechanism includes a special muscle, 
