"This is an old one," she said, turning it over with a pencil. "I don't think they've made this model for years."

"What is it?"

"It's a ventriculo-peritoneal shunt. They're implanted for the treatment of hydrocephalus."

"Hydrocephalus?" I knew the term, but was surprised to hear her say it. What other misfortunes would I learn about this child?

"It's commonly known as 'water on the brain,' but that's not real y accurate, although that's a literal translation from the Greek, hydro being water, and cephalus being head. Cerebral spinal fluid is constantly produced in spaces in the brain cal ed ventricles. Normal y it circulates through the four ventricles, and flows over the brain's surface and down the spinal cord. Eventual y the CFS is absorbed into the bloodstream, and the amount of fluid and pressure in the ventricles stays within acceptable limits.

"But if drainage is blocked, fluid wil accumulate, causing the ventricles to swel and press on the surrounding tissue."

"So hydrocephalus refers to an imbalance in the amount of CSF produced and the rate at which it drains from the ventricles."

"Exactly."

"And, as the CSF builds up, it causes the ventricles to enlarge and the pressure inside the head to increase."

"You've got it. Hydrocephalus can be acquired or congenital, which is not to say hereditary. The term simply means the condition is present at birth."

"I found the shunt in a normal-looking skul . Doesn't hydrocephalus result in increased head size?"

"Only in infants, and only if left untreated. As you know, with older children and adults the bones of the skul are already formed."

"What causes it?"

"There are lots of reasons for inadequate CSF drainage. Prematurity puts an infant at high risk. And most babies with spina bifida have hydrocephalus."

"Spina bifida involves a neural tube defect?"

"Yes. The problem occurs during the first four weeks of gestation, often before the mother knows she's pregnant. The embryo's neural tube, which develops into the brain, spinal cord, and vertebral column, fails to form properly, leading to varying degrees of permanent damage."

"How common is it?"

"Entirely too common. It's estimated that spina bifida affects one in every thousand babies born in the United States, and about one in seven hundred and fifty born in Canada."

"I recovered no vertebrae, so I have no way to know if my young lady had spina bifida."

Russel nodded in agreement, then continued her explanation.

"There are many other causes of hydrocephalus besides spina bifida." She ticked them off on her fingers. "It can result from brain hemorrhage. The inflammation and debris resulting from brain infections, such as meningitis, can block drainage pathways. Tumors can cause compression and swel ing of brain tissues and result in poor drainage. So can certain types of cysts. And hydrocephalus can be familial."

"It can be inherited?"

"Yes. Though that's rare."

"So where does the shunt come in?"