1212 VESTIGIAL AXD ABNORMAL STRUCTURES 



the tliird montli, however, the lower part of the spinal cord ceases to develoj) in 

 the same matter as the upper part; at birth the functional part of the spinal cord 

 terminates at the third lumbar vertebra, and in the adult it ends at the level of the 

 second lumbar vertebra, the terminal or caudal portion of the cord, which is better 

 developed in many vertebrates, being represented in man b}'- the tilum terminale. 

 This retrogression of the lower part of the cord is not, however, contined to man, 

 for it is present also in other mammals, and it is particularly well marked in the 

 hedgehog, the filum terminale commencing in this animal in the anterior part of 

 the thoracic region. 



In the human adult the filum terminale is about 23 cm. (9 inches) lopg, and 

 it is divisible into two parts — an upper, about 14 to 15 cm. (6 inches) long, which 

 lies in the subdural space, and a lower part, about 7 cm. (3 inches) long, which is 

 closely invested by a covering of dura mater. The first part consists, at its com- 

 mencement, of an outer layer of nerve-fibres, the fibres of the coccygeal and acces- 

 sory coccygeal nerves (thirty-first, thirty-second, and thirty-third pairs of nerves), 

 a small amount of grey matter, some substantia gelatinosa centralis, and the lower 

 part of the central canal. The central canal terminates about 4 cm. beneath the 

 conus medullaris, the grey matter al^out 8 cm. beloAV the same point, and the 

 remainder of the first part of the filum consists, therefore, merely of a few nerve- 

 fibres and connective tissue. The nerve-fibres of the thirty-second and thirty-third 

 pairs of nerves are quite functionless, and they disappear after a very short course 

 in the second part of the filum, the lower portion of which consists entirely of 

 strands of connective tissue more or less separated from each other, and attached 

 at intervals to the back of the coccyx. 



The pineal body. — This structure, which appears at an early period in the 

 development of the foetus, is a diverticulum from the roof of the primitive fore- 

 brain, or thalamencephalon; it grows backwards, and in the adult lies between 

 the splenium of the corpus callosum and the anterior quadrigeminate bodies, 

 embedded in a fold of pia mater. It never attains a high grade of development in 

 man, nor, in him, does it appear to possess any special functions, but in some of the 

 lower vertebrates it appears to serve the purpose of an unpaired median eye, and, 

 reaching the surface of the body, it is en^edded in the parietal foramen in the 

 skull wall. In the higher vertebrates, however, including man, it is displaced from 

 a supertieial position liy the great development of the cerebral hemispheres. 



The pituitary body. — The pituitary body is connected in the adult with the 

 floor of the third ventricle, and it is embedded in the pituitary fossa of the 

 sphenoid bone. It consists of two lobes — an anterior, of somewhat glandular 

 nature, and a posterior, in which remains of nervous structures can sometimes be 

 recognised. The posterior lobe is formed by an outgrowth from the floor of the 

 thalamencephalon, and in man and other mammals the anterior lobe is a diver- 

 ticulum from the roof of the stomatoda^um, or primitive mouth, but in the lower 

 vertebrates the anterior lobe is formed by a diverticulum from the olfactory 

 depression, and consequently its origin in man is a secondary one. 



The history of the pituitary body has not yet been fully elucidated, but it is 

 ])elieved that it represents the remains of the ancestral mouth, and that the mouth 

 which is now formed has been developed by the fusion of a pair of gill clefts. 



THE EYE 



The canal of Cloquet or Stilling. — Almost in the axis of the eyeball there is 

 a fine canal passing througli tlie vitreous body from the ojjtic papilla to the back of 

 the capsule of the lens; this is the canalis hyaloideus, or the canal of Cloquet or 

 Stilling. It is lined by a fine homogeneous membrane, and in the adult it may 

 possibly serve as a lymph passage, but in the foetus it transmitted the capsular 

 branch of the central artery of the retina from the optic papilla to the back of the 

 lens capsule, which at that time was a very vascular structure. 



