CH. L.] COMPAKATIVE PHYSIOLOGY OF THE BRAIN 713 



but with the increasing complexity and delicacy of the movement ; 

 (compare in fig. 498 the relative size of the areas which control the 

 trunk muscles and the finger movements). It is just these move- 

 ments which are most affected by a cortical injury, and which exhibit 

 least recovery ; in the upper limb, for instance, the shoulder muscles 

 will be the least, and the hand the most, paralysed. 



On the sensory side of the cortex, vision alone can be analysed 

 with sufficient accuracy. The* lower the animal in the series, the 

 more readily can its actions be controlled by sensory impulses which 

 have not passed through the cortex cerebri. A decerebrated bony 

 fish can distinguish colours, a frog can catch flies, even a pigeon will 

 select its perch, though it takes no notice of food or of people who try 

 to frighten it. A dog similarly operated on is practically blind, 

 though it will blink at a bright flash of light. In the lower animals 

 the impulses pass in to the primary visual centre which acts as the 

 centre for the reflex; the higher we ascend the animal scale, the 

 path via the cortex becomes more permeable, of greater value or 

 even indispensable, and the reflexes through the lower centres of less 

 importance; not only so, but there are subdivisions of the visual 

 cortical area, which correspond to different regions of the retinae. 



In the fishes which have no cortex cerebi, the optic lobes, analogous to. the C. 

 quadrigemina, are the centres for vision. In some fishes, a small number of the 

 fibres of the optic nerve pass into the geniculate body, which forms a cell station on 

 the road to the posterior region of the cerebrum, where a primitive cortex begins to 

 appear. On ascending the animal scale, this group of fibres becomes more and more 

 abundant, and this part of the cortex becomes more elaborate in structure. When 

 we reach the monkeys, this part of the brain is cut off from the rest to form a dis- 

 tinct occipital lobe by the parieto-occipital fissure, which is frequently called the 

 Affenspalte (ape's split). At first this lobe is smooth (fig. 497, p. 685), but as the 

 great parietal association centres get larger with increase of intelligence, the visuo- 

 sensory area is pushed back, and is thus thrown into folds. In the highest apes, 

 and in the lower races of mankind, a good deal of the visuo-sensory sphere is still 

 seen on the external cerebral surface ; but in the higher races, most is pushed round 

 on to the mesial surface (area 4, figs. 506, 507, p. 696). This calcarine area is better 

 named the striate area, because it is characterised by the white stripe called the line 

 of Gennari (see p. 689). 



Some animals have panoramic and others stereoscopic vision. The former 

 (mainly vegetable feeders) have eyes set laterally; each eye receives a different 

 picture, and the decussation of the optic nerves is complete ; each eye sends 

 impulses to the opposite hemisphere. Animals with stereoscopic vision have the 

 eyes, as in man, in front, and the optic axes can be converged so that an object is 

 focussed with both eyes. This becomes necessary in carnivora, which have to catch 

 moving prey; the more complex the movements of the fore-limb, the greater 

 becomes the necessity for fixation of the eyes to guide them. In such animals each 

 visual area corresponds with the same half of both retinae, that is, with the opposite 

 half of the visual field ; the lower half of each area corresponds with the upper half 

 of each half field of vision, and vice versa. The appearance of the macula lutea (with 

 cortical representation in both hemispheres) in the primates is the culminating point 

 in visual development. 



A man or an animal who loses both eyes is blind, but in time manages to find 

 his way about. This is not the case when blindness is produced by removal or 

 disease of both occipital lobes ; here, the sense of orientation is lost also, for the 

 association of many essential sensory and motor impulses is then impossible. 



