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PATHOLOGICAL ANATOMY 17 pia in the lateral and posterior parts of the cord is free from round cells, the infiltration around the vessels stops at the periphery of the spinal cord and does not involve the pia. It is noteworthy that in the dorsal and cervical regions, where the implication of the pia on the external aspect of the cord generally is either very slight or completely absent, the infiltration of the pia is still most obvious towards the bottom of the anterior fissure. Considerable infiltration may occur here and around the entrance of the central vessels, although the pia elsewhere escape. These facts are of importance, for they enable us to realize the relation of the changes in the pia to those in the spinal cord.

On microscopical examination, the pia is seen to be infiltrated, and its blood vessels are found to be dilated and congested. In- flammatory changes have been observed in the pia mater of the brain, also. The implication of the pia mater in infantile paral- ysis was noted first by Dauber, and subsequently by Bickel (Roeder and Schultze). Wickman established, and Harbitz and Scheel and others have since corroborated, that changes in the pia are a constant phenomenon of this disease.

The affection of the pia mater is of special interest, as it ex- plains the meningeal symptoms, which, in the initial stages of otherwise typical spinal infantile paralysis, are sometimes ob- served. The affection of the pia is also the basis of the meningeal form of Heine-Medin's disease.

As a rule, the dura is not implicated. Harbitz and Scheel suc- ceeded in demonstrating a slight cell infiltration of the dura mater in only two cases.

Spinal Cord. — The changes in the spinal cord are most evident in the interstitial tissue and in the vessels. Of these changes Rissler has written a classical description. The vessels are dilated and congested. The vascular changes are especially marked in the veins but are not confined to the larger vessels if the process is at all advanced. The capillaries are often greatly distended and tor- tuous. Siemerling and Matthes allege that a formation of new capillaries occurs. The contents of the vessels are red blood cor- puscles.

Wickman, Harbitz and Scheel, and Strauss, in spite of most careful search, failed in all their numerous cases to find any evi- dence of thrombosis, or embolism, in the first stage of the disease;