512 COMPARATIVE ANATOMY. 



developed in some species of Trigla (cf. Fig. 289, B), as is a smaller 

 number of them forming the exceptionally short spinal chord of 

 Orthagoriscus, etc. (A). 



As the size of the spinal chord is influenced by the masses of 

 nerves which are given off from it, we find that in the four higher 

 classes of the Vertebrata the great development of their extremities 

 and the large size of the nerve-chords that go to them are correlated 

 with the increased development of the size of the spinal chord in various 

 regions. In this way the cervical, or thoracic, and lumbar enlarge- 

 ments are formed ; they are very large in some cases (e.g. Chelonii 

 and Aves). The primitive medullary cavity, which persists as the 

 central canal, remains open in the lumbar swelling of Birds, and thus 

 a sinus rhomboidalis is developed ; this is similar to the sinus which 

 is always found in the medulla oblongata. It is found for a short 

 time in the embryos of Mammals also (Fig. 279, a). 



As a rule the spinal chord extends through the whole of the 

 spinal canal; but in the Amphibia (Anura) and Aves, and most 

 markedly in many Mammalia, it is pressed more forwards, owing to 

 the unequal development of the enclosing and enclosed parts, so that 

 the nerves which are given off from it for the hinder parts of the 

 body, have to run for some way in the spinal canal before they get 

 to their orifices of egress. 



c) Investments of the Central Nervous System. 



§ 38k 



As the cavity of the skull is adapted to the brain, which it 

 encloses, this latter at first fills up the cranial cavity. The same is 

 true of the spinal chord and spinal canal. The surface of the whole 

 of the central nervous system is separated from the walls of its case, 

 which are formed by the skeleton, by parts which either belong to 

 the skeleton, or to the nervous system, or which are interstitial in 

 character. These form the coverings of the brain and spinal chord. 



The periosteal investment of this skeletal cavity develops the 

 dura mater. In the lower divisions this membrane appears to be 

 a mere periosteal (or perichondrial) layer; it is not much larger till 

 we reach the Reptilia, where it first acquires the character of an 

 independent formation. In the cranial cavity of Birds it forms a 

 process between the hemispheres of the prosencephalon (falx 

 cerebri), and this is found in most Mammals also, where it is accom- 

 panied by a process — the tentorium cerebolli — best developed in 

 the higher orders, which projects between the cerebellum and the 

 posterior lobes of the prosencephalon. In many Mammalia (Car- 

 nivora, Perissodactyla, etc.) the tentorium is ossified. The spinal 

 portion of the dura mater is less peculiar in character. In the Mam- 

 malia it is separated from the periosteum as high up as the occipital 

 foramen, and forms a sac, which loosely envelopes the spinal chord. 



